So I just did the first oil change and noticed something that caused my eyebrows to raise... I have a 1968 C3 (non-matching) with a small block 350 from a 1971 El Camino (that's what it appears to be based off of casting and stamped numbers).

I don't know what weight oil the previous owner was running in the engine but after the oil change now it idles around 50# when it was much lower than that before (I think it idled between 20-30# before the change). I checked the fill level and saw it was slightly overfilled so I let out the excess until it was right at the full line on the dipstick and checked the oil pressure gauge again - maybe dropped 2# if that.

My main questions are thus:

1) Is that too high of a pressure to run safely? (Mechanical gauge, 70# max)

2) Is that a normal behavior for the mechanical gauge to not show much of a pressure drop after draining some oil from the system?

3) Does that pressure level still seem too high and should I change it out to a different weight before driving it? (Have only had it idling to check pressure readings - a local Auto Zone employee recommended a "race oil" 50 if I want to run it hard.)

Thanks in advance for your advice!

 

Did you allow the engine to warm up? When cold, the oil pressure will always be significantly higher, because the oil is thicker. But, 50psi won't hurt anything. If it stays that high [at idle] when the engine warms up, something would not be right. And, if all you changed was the oil and the filter, I would suspect a problem with the filter.

 

If the filter is not a problem then that means you have good tight bearing clearance. I would not use 50 weight oil unless you have low oil pressure when hot. 10# for 1000 RPM is an excepted level. Straight 50 weight has no cleaners or additives for motors. If you end up having to use 20W50 but only as a last resort. You should be OK.

 

Thanks for the replies!

I've got the car idling right now and I'm going to take an easy drive around the subdivision to see if the pressure level changes once the motor is nice and warmed up. The filter is an STP - it was a package discount deal with the oil and since I didn't know if I had the right oil I figured it couldn't hurt. I guess that may not have been the best mentality. :P

 

My .02 is that STP filters are one of the worst out there. You can practially crush them in your hands like an alum. beer can. If you put them on too tight you will have a *itch of a job getting it off. My last experience with them required using the screwdriver as the removal tool. On your next change, just buy a AC or a Puro unit. These are just my opinions FWIW. I would also recommend you use Delo 400 15W-40.

 

If you want to give choices of oil viscosity some thought, consider the following.

Most everyone has their own idea as to what is the best motor oil brand and viscosity to use. For me, I run 5W30 in all my Hotrods, which all use roller lifters, with no issue at all (I run the factory called for 5W20 in new daily drivers). My Hotrods range from a 600 HP Supercharged small block, to an old school 781 HP, 710 ft lb, 540ci BBC street/strip motor, intentionally setup with .003" clearance on the rods and mains. And it survived its dyno thrash just fine without tossing a rod, without smoking, without pumping the pan dry, still shows 3% COLD leakdown (with conventional rings and .021 top ring end gap and .027 second ring end gap), and has had no problems at all, so it’s all good.

And even with the relatively thin 5W30, the 540 has a hot idle oil pressure of around 30 psi at about 1,050 rpm. And once it reaches enough rpm to reach the oil pump’s relief valve setting, it maintains a nice steady 80 psi hot oil pressure all the way to the redline shut down point, without the pressure ever dropping back any at all. It’s able to make that kind of oil pressure even with extra oiling holes for the timing chain and gears, as well as an extra oil hole to directly pressure feed the distributor/cam gear mesh point. So, that is living proof that you do not need thick oil to maintain a good amount of oil pressure with decent sized bearing clearances, as long as you run a high volume oil pump. And I am running a Titan gerotor high volume oil pump that moves a lot of oil.

As long as you run a premium quality full synthetic 5W30 that has outstanding film strength (more on that below), you'll be good to go. But I would NOT use and I would NOT recommend plain old petroleum 5W30, because it is not even in the same league as the latest and greatest full synthetic motor oils. I've run 5W30 Castrol Edge, 5W30 Royal Purple street oil, and 5W30 Royal Purple XPR Racing oil. I like them all just fine and have no complaints, other than the RP XPR costs $16.00 per quart from Summit. And you have to order it because regular Auto Parts stores don't carry it.

5 is the cold start-up viscosity rating of the 5W30, and 30 is the hot or operating temp viscosity rating of the 5W30. And a 5 viscosity cold rated oil will flow quicker and better to vital engine components on start-up, than heavier a 10, 20 or 30 viscosity cold rated oils. And start-up is where 90% of all engine wear takes place. So, when it comes to cold start-up, 5W30 is ALWAYS better than 10W30, 10W40, 20W50 or straight 30 wt (which has THE worst cold flow capability of all the viscosities listed here).

Many folks think that pressure = lubrication, but that is simply not the case. Pressure is only a measurement of resistance to flow. Flow = lubrication. And lubrication is what is used to separate moving parts, and keep them from touching. Pressure and flow are inversely related. A thicker oil will increase pressure, while decreasing flow. And a thinner oil will increase flow, while decreasing pressure. A 30 viscosity hot or operating temp rated oil will flow quicker and better to those vital engine components during normal operating temps than a thicker 40 or 50 (from 10W40 or 20W50) viscosity hot or operating temp rated oils. Bottom line: Flow is the only thing that lubricates, and you get more flow with thinner oil.

Increased flow also has another important advantage. An engine's vital internal components are all DIRECTLY OIL COOLED, but only INdircetly water cooled. And thinner oil will flow more freely, carrying away more heat, thus providing better cooling for those vital internal components. Of course that extra cooling is even more important in high performance engines.

Here are some comparison numbers for you from an 830 HP road race engine on the track:

15W50 oil = 80 psi = 265* oil temp

5W20 oil = 65 psi = 240* oil temp

Here you can see how the thicker oil flowed more slowly through the bearings, thus getting hotter and driving up bearing temps. If an engine is running hot, use a thinner oil to increase flow and increase cooling.

And then for the icing on the cake, thinner oil also typically allows an engine to make a little more HP than thicker oils. In fact, Royal Purple offers their XPR 0W5 which is their lowest viscosity engine oil designed for the most competitive classes such as Pro Stock, Pro Stock Bike, Comp Eliminator and NASCAR Cup (qualifying). This ultra low viscosity provides the most horsepower possible by keeping parasitic losses to an absolute minimum while providing unparalleled protection. So by comparison, a much more conservative 5W30 motor oil seems downright thick.

So, when you combine the extra flow/lubrication, the extra cooling, and a little extra HP that thinner oil provides, its easy to see how 5W30 motor oil is a better viscosity choice than the thicker 10W30, 10W40 and 20W50 motor oils that cannot flow as quickly during cold start-up, cannot cool as well, cannot lubricate as well, and cannot make quite as much HP.

Yet many folks aren’t comfortable with thinner oils because they think that heavier viscosity oils offer better protection against metal to metal contact. But when things go bump in the night, it isn’t viscosity that prevents metal to metal contact. It is oil film strength that prevents metal to metal contact. And an oil’s film strength is determined by its additive package. And that is the reason oil companies can offer watery thin viscosity racing oils that still provide excellent wear protection. Also, different oils of even the same viscosity, can perform very differently, depending on their additive package. But you can't buy an oil company's "additive package", because they all regard them as highly guarded proprietary secrets.

Base stocks for like type oils are very similar. So, what separates one brand's oil performance from another brand's performance, is the difference in their additive packages. An example of this is the full synthetic Castrol Edge. When it first came out, lab tests showed that it provided 8X better wear protection than Mobil 1, even for the SAME viscosity. Simply put, the tests showed that Castrol's additive package is far superior to Mobil's additive package. It's for the same reason that Royal Purple oils offer such outstanding wear protection, because of their proprietary "Synerlec" additive that only they have. I favor these two oil products because of their impressive film strength capabilities.

I exchanged emails with one of the Motor Oil Engineers at Royal Purple, and asked the questions below. His answers are immediately below the questions:

Q 1 - Is an oil's film strength determined by the oil's base stock or its additive package?

A 1 - An oil's film strength does not vary greatly with type of base stocks used in motor oils. An oil's ability to stop metal to metal contact under extreme pressure's is obtained through its additive package technology. The base stocks determine physical parameter's such as viscosity, flash point, and to a great extent - pour point, etc.

Q 2 - Is it viscosity or oil film strength that protects an engine from metal to metal contact?

A 2 - Oil film strength and viscosity help separate metal surfaces and prevent contact. Viscosity alone will protect only up to a certain point, before either the load or the temperature would allow metal to metal contact. That's when the oil’s film strength from its anti-wear and extreme pressure additive package takes over.

Q 3 - Does going to a higher viscosity offer any increase in engine wear protection?

A 3 - Viscosity is a measure of the oil's resistance to flow at a given temperature. Increasing viscosity does not necessarily increase wear protection and running too viscous an oil for the application can actually decrease protection as one of the functions of an oil is to remove heat and combustion byproducts. Running 20W50 in an engine designed for a 5W30 results in reduced oil flow through the engine's bearings, so less heat can be transferred away from the bearings due to that decreased oil flow.

Its not surprising that folks who were raised using plain old petroleum 10W40, 20W50 and straight 30 wt back in the day, are hesitant to change over to the new technology of the vastly superior and thinner full synthetics we have today. People are creatures of habit and like to stay with what they are comfortable and familiar with. But, you know what they say, those who can’t or won’t adapt and change, become extinct.

Today’s thinner full synthetics are by far the most superior motor oils we’ve ever had available. So, users of the older thicker viscosities may want to give the newer and thinner full synthetic oils some serious consideration. Then they too can enjoy all the benefits that come with using thinner oils. At the end of the day, folks have to decide for themselves what oil brand and viscosity they want to run. But personally, I would not run anything thicker than 5W30, as long as I can get top of the line premium full synthetic oil, with outstanding film strength, in that viscosity.

 

Just got back from the test run around the area with the motor fully warmed up. Cold idle is around 50psi, warm idle is around 29psi. I feel better about the situation now. Thanks for all the input, guys!

TWINRAY, agreed, I'm not a fan of the STP filters either but it was a package deal - hope I don't have an issue removing it during the next change... Next time, package or not, I'm going to buy something else.

540 RAT, thanks for the information! I feel much more educated now and it appears I have some research to do before I do the next change! Due to the lack of information known about the motor before this change, I used less expensive and more common components to see what the results would be. As I slowly build the engine up I'll make sure to factor in the information you shared.

Thanks again!

 

right on!

 

Outstanding, Rat!!

 

Thanks Rat for all the info. I now know alot more about oil than I used to. Definitely saving this on my 'puter.

 

really good info 540 RAT. everything he stated makes since. Thanks.

 

I run 10W30 in a 406 6200 RPM drag engine. The reason for that is I had ran 20W50 and thought it may have been too "thick' as I did turn a bearing. Lucky for me the engine builder was in the stands when we laid down a 11 second pass and repaired the engine free of charge. I would like to learn more of viscosity under heating conditions.

With the 10W30 the oil will get real thin quickly with heat. I have a low pressure sensor tied to a 'red' indicator light and will at times after a 1/4 mile pass see it dip below the 15 psi setting. A quick trottle stab will bring it back up but I do not let it idle.

I would think again of a "thicker" oil with properties next time we rebuild that stump pulling small block.

Dano

 

Excellent article RAT. Believe it or not I use 0W-30 synthetic in my DD during the cold winter here.

 

It was enjoyable to read 540rat's correct evaluation of the oil numbers. Way to many folks fail to understand those number meanings. Some of the newer cars have a 0w20 or more. If the warm weight was 0, Houston we have a problem.
That permits lighter starter and smaller battery.
I read here often people who address low oil pressure with higher vicosity numbers. Most of the time the lower numbers, with regard to oil pressure, are a sign af either incorrect pressure numbers ie: bad gauge, or a looming issue on bearing clearences/ crank wear/ rod distortion.