You are almost correct, but the temperature scale doesn’t slide. The W number is always taken at 32F and the upper number is always taken at 212F. Viscosity plots as a straight line on semi-log graph paper. That’s a slight fib because it’s never a dead straight line, but it’s close enough for this discussion. The black lines on the plot at the bottom of this post show the mid points of the ranges for 5, 15, 30, and 50 single grade oils. The horizontal axis is temperature, spaced in normal, linear fashion. The vertical axis is viscosity in centistokes, spaced in logarithmic fashion, hence the name semi-log since only one axis is plotted logarithmically. The red lines are multi grade 5W30 and 15W50. To plot them, you mark an X on the appropriate lines at 32 (for the W number) and 212F (for the upper number) and connect them. The result shows that in a multi-grade oil like the red lines, the viscosity does not change as fast with temperature changes as it does in a single grade oil like the black lines.

For additional background which you did not ask about, but might find interesting, prior to the 1970’s, almost all oils were single grade dino oils. The exception was Pennsylvania grade oil such as Pennzoil and Quaker State. For reasons best known to Mother Nature, oil deposits in Pennsylvania had by far the highest concentration of isoparaffins of any oil every found anywhere in the world, and isoparaffins have very high VI (viscosity index, a measure of how well an oil resists viscosity change with temperature). So Pennsylvania grade crude oil would make a 10W30 multi grade oil right out of the ground without use of VI improvers. The Pennsylvania grade crude oil is now pretty much gone, so Pennzoil and Quaker State (now owned by Shell) had to find other ways to make multi grade oil. There are two: VI improver additives and use of synthetic base stock, both developed in the 1970’s.

Of the two current day methods, synthetic base stock is better because it will generate a 25 spread in W and upper viscosity grade numbers (like 5W30 with its spread of 25) without using VI improver additive. VI improvers have the problem that they shear with time. The best way to visualize that is by the octopus analogy. VI improver molecules look like an octopus. When cold, the tentacles wrap tightly around the body, so they flow normally through the oil. But when they heat up, the tentacles extend, interfering with flow, and thus raising viscosity. So that counteracts the tendency of the oil to thin out as it heats up. Trouble is, the tentacles can shear off, and once they do, the VI improver molecule has lost its ability to prevent the oil from thinning out with rising temperature. So any multi grade dino oil, and any synthetic with spread between W and upper number greater than 25 (such as a 0W40 with spread of 40) may lose viscosity at high temperature as it wears. Hope I answered your original question and that you found the VI improver info of interest.

 

Im just tired of people with zero real world experience throwing out bad information that causes people to ruin their engines or their cars....ive watched it on the garbage facebook groups and now it has migrated here.... We get new guys asking questions that could be answered with an advanced search and actually get the correct solution listen to some clown that tells him that he should do a body off to replace a dash bulb or something just as ridiculous. Im here to learn, help, give good advice and learn some more. Watching people get screwed isnt fun for me and its getting worse. Some of us dont have big bank accounts or drive new cars every year so we have to learn to fix what we have.
Frankly ive become very disappointed with what is happening here and feel sorry for a lot of the new guys that are just starting to learn about engines or corvettes
Sorry Air D youre on your own now....hope your engine survives

Last bit of info.....for the irrelevant discussion above..whats the bearing clearances for what engine all the bullshit information is garnered from....its certainly not a gen 1 SBC or a gen 4 BBC...clearances are pretty big in those motors ie our motors
Vs a new german or japanese motor

 

I have already admitted that I do not have real world experience with engine mods, racing, or rebuilding. I’ll add that I do not have any direct experience with 1979 L82 Vettes. But oil selection guides from major oil company websites, which are drawn from original GM recommendations, all say either 5W30 or 10W30 for street use in that car in normal climates. So I can’t find any evidence from GM or oil company data that the clearances in that car’s engine are enough higher to matter. I’ll also admit that in 1979, most oils still had zinc which helps flat tappets, so either finding a street grade oil with high zinc or using a zinc supplement is probably wise. But I continue to believe that going to 15W50 to get the zinc is unwise for the reasons outlined in my various posts which you say are irrelevant. So I guess we’re stuck with disagreeing, and the OP and other readers will have to decide for themselves.

 

I run 15-50 synthetic because I live in Florida and it gets what one might call hot. Synthetic oils flow easier than conventional oils. Also. GM did indeed recommend the weight I run in hot climates

 

I have never built an engine to GM spec. I build them for .002-.0025 on the rods, and .003-.0035 on the mains....for most applications. GM specs are much tighter than that. So your whole basis of your conversation is shot to **** almost immediately.......
Bearing clearance and lifter restriction are the major two points of controlled leak in most V8's of the pushrod regime.....Pro Engine builders open up the clearance from GM spec to improve flow, which reduces temperatures and keeps a good meniscus of oil on the bearing to handle loads.....
If I set a Gen 1 small block to .0025 on the rods and .003-.0035 on the mains, with a flat tappet lifter.....with 15w40 oil I will have 30psi at idle and 60 psi at 5000rpm.....if I go to 5w30 those numbers go down to 25/55....if i run 20w50...those numbers go up to 35/65......
Centistoke ratings are great as a comparison for hot and cold flow but says jack and **** about the oil flow in my Small Block compared to say, a Honda CBX 1000 6 cylinder bike engine that runs .0015 rods and .0015 mains as a sweet spot (These are the clearances of the last one I built a month ago)....so the Centistoke rating is not a viable number to gauge what oil you should use from engine to engine.....it is but a guide....loose clearances require thicker oils and tight clearances require thinner oils......because it is about the flow....and the flow creates the cushion.....cushion is how bearings stay alive. It is a balancing act......so your ranting about Centistoke numbers is pointless here and is a better conversation for 2 Car Guys or some brainless overwrought auto "enthusiaist" platform.....go pitch it to new Jeep owners.....they love that kinda ****.....
Pro engine builders give two ***** about centistoke ratings......they care about Oil Pressure and Oil Flow.......that's the tell.

Jebby

 

I may be missing something, but I don’t see any serious disagreement between us. As far as I understand this thread, the OP’s car is stock, to typical GM clearances. Obviously, if you have modified the engine to looser clearances, you need heavier oil than GM recommends. I think that in a stock engine on the street with frequent warmup cycles, choosing the right grade of oil for the frequent temperature cycles it goes through is the most important factor because I view the clearance issue as being chosen by the factory and beyond my control. But I concede that if I were building engines and could choose the clearances myself, I’d probably think like you do. So it seems to me like what differences we have can be explained by the fact that you view clearances as something you can choose as you build the engine, and I view them as being fixed at the factory. That in turn means we are both correct for the applications we are talking about. Am I still missing something?

 

I don’t have an owners manual for that car so don’t know details of the factory recommendations, nor do I know typical oil temps for that car at various ambient temperatures or even whether that car has an oil temp gauge. What I can say is that as far as I know, pretty much all GM engines leave the factory with clearances that like oil to be in the range of 5-20 centistokes. With 5W30 that translates to about 185-265F oil temp. With 15W50, that increases to about 220-300F. If you don’t need the added high temp end above the 265F of 5W30, you have unnecessarily paid the price of a longer warmup period at higher viscosity. Even with a nice warm 75F morning Florida startup, both oils are too thick, but the 5W30 is less bad at about 130 centistokes versus about 350 for the 15W50.

 

oil weight and bearing clearances go together, recent Mustang 429 build the Morel hydraulic roller lifters specified engine clearances to be set up for 10W30 for best results and we did tighten it up a touch , during the same time did a 427SBC employee car with Morel lifters and this engine is set up for 20w50 summer or 10W40 in the cold .... On new street builds I never use synthetic oil until they hit 5K miles or more for ring seal and in these cases we break in with Driven break-in oil and after 350 miles switch to Valvoline VR1 non-synthetic until it least 5K miles

 

How can you sit here and say to use one oil over another and have zero idea of 50 yearold engine design. Not to mention adding in engine wear and say 10-30 is fine...back in the late 70s 10-40 was a winter recommendation. 10-30 in the summer for newer cars. Some used sraight 30 weight or 40 weight in the summer. Older cars always got straight 40...most all smoked. None usually made it past 100k miles.
You are recommending oil to a guy with a 50 yo car and engine and telling him oil for a 2015 Toyota Coralla is a good choice. You speak of theory, not practically.
You should not be recommending anything.

 

Always subscribe to oil threads. Always. This one has been great!

Only 81 and 82 cars came with an (optional?) oil temp gauge, though it can easily be added to any 77-82 car, and replaces the useless clock.
https://www.zip-corvette.com/81-82-o...ding-unit.html

The oil pressure gauge in the car almost certainly reads incorrectly. I finally got around to testing them against a mechanical gauge, and neither of my oil pressure senders work. In the OP's 79, there is a plugged port behind the distributor that can be used for an independent oil pressure gauge (this is the stock location in my 80).

It seems that any science-based oil recommendation should measure and consider these two variables.

Otherwise, for a stock, GM-built Gen 1 SBC, with stock bearings, flat tappets, and isn't run in the cold, wouldn't Mobil 1 10W-30 synthetic with some ZDDP be the correct answer for the OP? That's what he already has. I wasn't aware of the VI improver additive issue, but I change my oil often enough that it may not be a problem for the Vettes.. I'll keep running my 0W-40 with ZDDP in the meantime, and add some proper instrumentation.

 

Yes, the most obvious fact......about 1 in 100 of these cars and engines at this point were built by GM.....most all have been rebuilt at this point. And the builder who chooses what clearance to run has free will.....not a GM spec.....so it may or may not be correct.....
My 2013 GMC Terrain 2.4l ecotec that I recently rebuilt had .0013 on the rods and .0016 on the mains.....spec oil is 5w30.....which is too thin for a hand built V8.....built to the specs I mentioned in an above post.....so like I said, centistoke as a recommendation is flawed here.

I generally ten foot pole oil threads.......but this was poking at me.......

Jebby

 

Did you get that exactly backwards?

Anyway, here's what's in my 79 shop manual. It's a bit different than the one posted earlier, and includes single-weight oils, and even 20W-50.



I believe that ratio for stainless-sleeved brake calipers. I absolutely do NOT believe that about these engines. I think there are a lot more original engines out there, including 100% of the SBCs in my garage.

 

I don't either, but I do have experience with a '76 L48.......almost 48 years worth.
When I did my 1st oil change in the summer of '76 I used Valvoline 20w/50 Racing. It's never winter driven, so why not...
That's all I used until Maxlife came out about 20 years ago. I switched to 10w/40 around 60k & I'm at 78k now.
My little untouched SB doesn't burn oil & it runs great......

 

I have posted mostly on fuels and lubes topics in C6 and C7 general forums. Lately, I’ve noticed the trending topics section and have checked oil related threads in other sections, which is how I got to this one. I’ve always tried to be careful about admitting what I didn’t know, and also admitting when I’ve made errors. I appreciate the attitude of Jebbysan, who disagreed with me and provided a reason, namely that most engines of that era have been rebuilt, so the factory specs and oil recommendations are meaningless. To the extent that’s true, then I stand corrected, but I also note that Bikespace believes quite a few are still original from the factory. I admit I don’t know, and also admit that I don’t know the degree to which engine wear has opened up the clearances. I also thank Bikespace for providing the owners manual recommendations that did indeed extend to 50 weight for high temperature operation as 7t9l82 said. Anyway, I now realize that a tech forum for cars of this vintage is a different animal than general forums for recent vintage. That said, while there’s more wiggle room needed for using a chart like in post 41 with older or modified cars, I still think it has value, and that value increases the closer you get to stock configuration and/or the years since 2000.

 

Good to hear. I drive an older car by some standards, a 2011 CTSV coupe with M6, bone stock except for the skip shift eliminator. But it's a late model in comparison to those in this thread, which is part of the reason I got into some static in this thread. Previously, I had bone stock C4, C5, and C6 Vettes, for a grand total of 250,000 miles in the three Vettes plus Caddy-Vette CTSV. I used 5W30 full synthetic in all of them with no issues.

 

of course...10w30 was mostly a winter use and the 40 was summer but the straight weight would help the really beat engines
Typing at 5am is a real challenge