As many have said, Dexos is a spec, not an oil. Dexos spec is not better than Mobil 1 or any other full synthetic, it’s really the reverse. An oil that just barely meets Dexos spec is deemed “good enough” by GM. Full synthetics will significantly exceed Dexos. The original “Corvette oil spec” was 4718M, built around original Mobil 1. Since that time, all oils have improved, and GM decided that its new Dexos spec, which applies to all its cars, was also good enough for Corvettes.

I'm getting redundant here, but remember, neither Dexos nor 4718M is a specific brand. They are both specs. To be labeled Dexos or 4718M, the oil simply has to meet those specs. While not universally true, almost all oils that meet 4718M will also meet Dexos. But quite a few oils that will meet Dexos will not meet 4718M. To meet 4718M, an oil pretty much has to be full synthetic, but a semi-synthetic (meaning a blend of synthetic and conventional base oil) that has a good additive package can meet Dexos. For most situations, Dexos is indeed “good enough”. But if you want that last tiny little bit of protection for very severe service, use an oil that is both Dexos and a full synthetic.

The GM-branded oil that is labeled GM/Dexos is a semi synthetic, not a full synthetic. That is why some people are willing to pay extra to get Mobil 1 in their “free” oil changes at the dealer. In terms of practical difference, unless you just run the bleep out of your car, or plan to keep if for 100,000 miles, there isn’t all that much for normal, even spirited normal use. Most people who use full synthetic (including me) do it from a mentality of “use the best, it doesn’t cost much extra”, rather than any actual need for it.

 

You are correct, but…… All oils have gotten better as additive package have improved. While I don’t have hard data to prove it, my educated belief is that if you compared today’s Mobil 1, whose base oil is a blend of group 3 and group 4 synthetics, you would find it is better than the original Mobil 1, whose base oil was all group 4 synthetic. For those who haven’t followed the many threads on this topic, group 4 are the original poly alpha olefin synthetics, while group 3 are what JerryU calls highly refined dino oil. But as additive packages have gotten better, the slight difference in group 3 versus group 4 base oil quality makes less and less difference in the quality of the finished oil. And now there’s even a new wrinkle. Penzoil’s platinum plus uses base oil from natural gas, which makes it group 3, but it is pure isoparaffin, which makes it essentially as good as group 4. So call it group 3+. The point is, once you are into a full synthetic base oil, whether it is group 3, 3+, or 4 makes little difference.

 

In know you are an oil expert and I am not so allows me to have fun speculating! However I managed an R&D group that made welding materials including fluxes. Fluxes are all very secret formulas, in fact a leading manufacturer got all their materials into the plant unlabeled as to the manufacturer and what it was, only a number was allowed on a container. (We were a division of a very large company that had a ferroalloy division and every year I got a list of that they bought!)
The real secret was not the ingredients, we could find that out (to some degree) but what mine and where in the mine did that batch of say manganese ore come from, for example. It was the very small impurities (like arsenic and sulfur) that were the issue not the basic chemical.
Therefore not being an oil expert I wonder compared to a man-made molecule (perhaps the Penzoil product made from natural gas meets that criteria) does a highly refined oil have the same level of residuals that may be harmful as a true synthetic molecule? Or does it depend on where the dinosaur died?!

 

Is it true that full synthetic oils have less detergent quality so that switching back and forth between types is not advisable?

 

Completely untrue on both counts. The additive packages in a top tier synthetic oil are more likely to be better (on all counts including detergency), and the notion of "can't switch back and forth" is about 20 years expired.

 

You welding guys are a crazy bunch with what you secrete. Better keep it a secret!

 

That's what I thought. You shouldn't trust the guy behind the counter at your local auto parts store.

 

You are certainly on the right track. If you look at dino oils, they start out containing isoparaffins, which are the good things, then normal paraffins which are wax, but easy to remove, then naphthenes which are moderately bad, then aromatics which are the worst of the main hydrocarbons, then trace elements like sulfur and nitrogen which are also very bad. To make a group 2 dino oil, you get rid of all the normal paraffins very easily, then almost all of the aromatics very easily, leaving you with the moderately bad naphthenes, and low levels of very bad aromatics, sulfur, and nitrogen. You then do a relatively low severity hydrotreating which doesn’t help at all with the naphthenes, but gets some of the really bad stuff. So in group 2 dino oil, you end up with a fair amount of moderately bad naphthenes, and low, but not zero levels of very bad aromatics, sulfur, and nitrogen.

The difference for the group 3 oils that the lawsuit allowed to be called full synthetics is that instead of the last step being relatively low severity hydrotreating, it is very high severity hydrocracking. That gets rid of all of the really bad stuff (aromatics, sulfur, and nitrogen) and a fair amount of the moderately bad naphthenes. So the only thing left other than the good isoparaffins are fairly low levels of moderately bad naphthenes. The group 3+ stuff from Pennzoil/Shell/Quaker State that is built up from natural gas doesn’t even have that small amount of naphthenes. I’d say there’s essentially no difference between group 3+ and group 4. There’s still a small difference between group 3 and group 4, but I’d say group 3 is about 80-90% of the way from group 2 to group 4.

So does that make it false advertising or bad practice? Depends on how you look at it. On one hand, group 3 isn’t quite as good as group 3+ or 4. On the other hand, it’s almost as good, a whole lot cheaper, and additive packages make that small difference even smaller. Ideally, they ought to label with group number, price accordingly, and let people decide whether they want to pay for the very small quality difference. But in the overall scheme of things, it certainly wouldn’t come close to making my list of most deceptive sales practices.

 

^^^ LDB, I think you should change your CF name to "akaBOB" as BITOG Bob.

It's truly refreshing to have yet another "oil" discussion when contributed to by someone with real information than the typical undereducated opinions. Your posts here have taught me some new stuff, and I've read quite a bit.

There seem to be only a few true oil experts here in the C7 section of CF, and you're now on that list in my book. I will reference this thread in future oil discussions here. Thank you for contributing.

 

I couldn't agree more. Lots of good stuff here. Thanks to all for contributing

 

Concur. My curiosity is satisfied. Thanks to all for your kind and patient explanations.

 

I agree, we could use LDB over on BITOG! (for those that don't know, I've been an administrator on that message board since it's beginning but I don't even have a fraction of LDB's technical knowledge, I'm just a big fan of all things motor oil related)

 

I'm not suggesting you SHOULD go double the mileage the OLI indicates, just that (based on engineering documents), there's a very large safety margin in GM's OLM.

 

The kernel of truth here dates from the very early synthetics. Dino oils cause rubber seals to swell, the early synthetics didn't. So switching from a dino oil to a synthetic in a well-used car could frequently cause the seals to shrink enough to leak.

Today's seals are made of better materials that don't swell as much to being with, and oil companies very quickly added additives to their synthetics to mimic that characteristic of dinosaur oils.

It's been decades since switching between dino remains and synthetics was as issue.

 

Great explanation from someone who knows! Thanks. Nice to know the additive package has improved so the Mobil 1 I'm using now versus from 1974 to 2000 is a better motor oil. I'll stick with it until something is proven better. Think I gave up on having an oil that gives less coking, which was my original concern. I'll get a new Vette after this is 5 years old and let someone else be concerned!

 

Thanks for the attaboys. Always appreciated. As far as BITOG, I probably don’t have time for another forum. I was originally a bit concerned about keeping busy in retirement, but that hasn’t proven to be a problem. So while I don’t have a Vette any more (after three of them, a C4, C5, and C6), the CTSV is a pretty close approximation with common heritage, and I’ll thus stay with the Vette forum for now.

 

I didn’t really think you were. I was simply warning people that might take it as a green light for extra long change intervals that to go past a year and 10,000 miles, you need the extra strength pH buffers of an oil that is specifically labeled long life. Full synthetic is not enough.

 

Yup, the DI coking issue is an interesting one. The better designed DI engines either have injector timing and geometry that pulses a bit of gas onto the intake valve while it is still open, and/or have a small port injector that delivers a portion of the gas that way. I know Vettes don’t use the latter arrangement. I don’t know if they use the former, but at least so far, GM DI engines are not noted as being problem engines from DI fouling. As far as the oil, a full synthetic with good detergent package should be a bit better on coking than semi-synthetic or dino oil. But it wouldn’t be a full solution. The important thing to me is that people don’t “solve” the problem by ditching their PCV systems. PCV is by miles and miles, the most important single pollution reducer, far more important than even the cats. We all have to breathe the air, and if you ditch PCV to “cure” DI coking, you are choosing to pollute our lungs rather than your intake valves, which in my opinion is a lousy trade-off.

 

LDB, Jerry and GlenE, thanks guys for your very patient explanations. It helps to explain why Mobil 1 is $5.00 a qt. I have used Mobil1 in all my vehicles since it came out. I now have a better understanding as to why to keep using it. Thanks again.

 

Dexos is a certification fee that companies pay to GM. It means NOTHING. Non Dexos certified oil that meets the Dexos specification (or exceeds in many cases) is all that is required. I'd also like to know how GM could pull the oil and say it was not Dexos certified since there is no magic elixir in Dexos certified oil.

Just another way for GM to pick the pocket of the average consumer.