oil question revisited

[Ken B.]

I was talking to a friend that has had corvettes for about 20 years. we were talking about oil usage in my new rebuilt motor.I was asking about using a synthetic oil and not conventional. He told me that i should not use synthetic until i get 2500 miles on the car. He said that things will not seat properly. Any truth to this. Thanks

[Ken A.]

Re: oil question revisited

No, but it will leak more.

[Michael W.]

Re: oil question revisited

Ken,

If you spend time searching for definitive answers on this question, you'll find as many opinions as you'll find authors.

What you won't find is good solid unbiased tangible proof of long term benefits of synthetic oils. I've been looking for at least ten years and have yet to find any. For every study that's published 'proving' things in one direction, there's another stating the opposite.

Stick with conventional motor oil, at least you're dealing with a known entity.

[Richard D.]

Re: oil question revisited

I believe most people feel that a break in period ranging anywhere from 500 - 5000 miles on conventional oil is better than breaking in with synthetic.

I also believe that most folks would agree that there ARE some benefits associated with using synthetic oils - lower internal heat generation, higher burn off temps, better cold flow characteristics, etc.

But, as Duke and many others have pointed out time and again, the best conventional oil for our older engines is a Diesel rated oil due to it's more robust additive package.

So, I use a synthetic diesel rated oil (carries both a gas and diesel certification), as I've had some "hot running" issues with my L-79, and felt better about the potentially better protection properties of syn. vs. conventional with this problem. I get a name brand at W-Mart in 4 qt. jugs @ around $4.00/qt (syn. diesel rated 5W-40). Since I only have to change the oil 1 time per year, the cost differential to me is acceptable.

As was stated, there's a million opinons on this stuff, so you just have to read as much technical info. on it as you can to make your own decision. The good folks on this board have posted this info many times, and it will certainly be in the good ole archives.

[Duke W.]

Re: oil question revisited

There is no legal or technical definition of "synthetic oil". It's become merely a marketing term.

Motor oils are made by blending the five base Groups - I, II, III, IV, and V with a suitable additive package for the intended service.

The higher the group number, the higher the viscosity index and oxidation resistance.

15W-40 CJ-4 is blended from primarily Group I with some Group II and/or III; 5W-40 is mostly Group II and III, and can be characterized as "synthetic" because the Group II and III base stocks are formed by "hydroprocessing", which is considered beyond normal refining, but I'm not sure if all marketers of 5W-40 CJ-4 claim it to be "synthetic".

For typical vintage Corvette service including cold starts at no less than 10 degrees F, 15W-40 CJ-4 is the best choice with oil change intervals of 5000 miles or one year - WHICHEVER COMES FIRST!

If you run more than 5000 miles in a year, but don't want to change the oil more frequently than annually, have to commonly cold start at below 10F, or run your car very hard and have high oil temperatures, then the added cost of 5W-40 CJ-4 is justifiable.

Duke

[Joe L.]

Re: oil question revisited

All new Corvettes manufactured since 1992 (except 92-93 LT-5) were factory filled with Mobil 1. If new engines required something other than synthetic oil, they would not have been factory filled with synthetic oil. I feel very confident the exact same thing applies to vintage engines newly rebuilt with modern internal wear parts.

[Duke W.]

Re: oil question revisited

Current Mobil 1 is rated SM and has less ZDDP than CJ-4, but prior to SM many motor oils were dual-rated for both the then current spark and compression ignition engine API service categories (SL and CI-4) and had about the same ZDDP concentration as current CJ-4. Today's Mobil 1 is very different in both base stock and additives compared to 5-15 years ago.

Everything changed two years ago when the new SM was limited to .08 percent phosphorous in winter grades of 10W or less. While CJ-4 also has a P limitation, it is 0.12 percent, which was the typical concentration of pre-SL spark ignition engine oils.

Modern SM oils are designed for modern engines with roller lifters, roller rocker arms, and catalysts. The reduced P helps extend catalyst life and the roller valve trains eliminate the sliding surfaces that need a higher dose of ZDDP for maximum longevity.

That's why CJ-4 is the best available current service category for vintage engines.

Duke

[Ken A.]

Re: oil question revisited

Duke & Joe are both right but for different reasons. If you've got a roller cam then use whatever you want. But if you have a pre "factory fill" engine then be prepared for leaks. The new engines have better seals, less crankcase pressure, and a leak proof design for the oil "pan" compared to the old design. I've tried Mobil 1 in a couple of engines & it worked fine except for the leaks at the front & rear pan gaskets as well as the timing cover. Replaced with "regular" oil & no more leaks.