Re: Synthetic oil?

Posted by Fish'n Jim On 2013/10/11 20:17:44

If you run any motor with less viscosity at the same temperature, you'll gain from less lost work/viscous friction(drag), regardless of it's source.
SAE Viscosity is measured @ different temperatures and why the numbers are different. A straight grade has a different curve than a multi-viscosity, 5-30, 10-40, etc.. 20-50 acts like 20W at the low measured temperature and like 50W at the higher, so it's not necessarily higher than 30, until the temperature raises. Weights are old traditional units and aren't necessarily indicative of large viscosity differences. The temperature effect is much more pronounced.
The additives modify the viscosity and film forming properties. (how they get multi visc.) These are depicted in the SAE letter designated service codes, on the can. Film has a relation to internal clearances, bearing materials, etc.
Detergents emulsify contaminates that result from combustion and thermal degradation, so they stay in the oil, so they can be discharged during changing. Without, they accumulate in the motor and oil. If you have no filter, use ND, better change oil frequently and/or be prepared to do pan cleanings. You can get problems running detergents in an old gunked up motor just as much as you can get a clean one gunked up from running ND - over a long time. It's depends how much.
It depends on what additives are in there and what motor's made out of, tolerances, dynamic balance, etc. if wear will be impacted.
Then you have the effect of the oil and additives on the non-metal parts to be concerned with. If you rebuilt with modern seal materials, you should be Ok to use modern oils. If the seals are petrified, it won't matter what oil you use, you'll get blue smoke and leakages. If you use the wrong fluids/oils, it'll attack the seal materials. I see references to adding ATF, etc. to cylinders to unstick, but that swells most rubbers. They used to sell it as an additive, so it can make an old seal pliable, but too much and seal's mush.
Diesels are another animal, the pistons are tin-plated, etc. Much higher operating temperatures / pressures / speeds / contaminates, etc. So no valid comparison to gasoline motor wear, etc.