Anecdotal historical facts about oil & turbos.
I'm not going to recommend a particular grade/multi-grade of oil. I want to share some facts about oil, and turbochargers from the last time we went through this fad in the 1970's.
Multiple weights are a compromise. Low viscosity when cold, yet relatively sustained viscosity when hot.
1) Engines run @ ~ 230F. Rings/pistons maybe ~400F. Turbocharger exhaust temperatures can vary between 600F & 1300F.
2) Turbocharger bearings are lubricated right between the intake & exhaust housings. I assume modern mfg.'s have solved the problem of continued lubrication of the turbo bearings after engine shutoff. That used to cause "coking" (essentially cooking/burning of the motor oil). It would ruin the oil, destroy the turbo bearings, and lead to premature demise of those 70's economy engines.
In the old days we'd allow the turbos to cool down, by idling/slow driving for several minutes before we shut off the engine. I cringe at the thought of automatic engine shut off at lights for fuel savings on a turbocharged motor.
3) Modern engines have tighter clearances and can tolerate thinner oil between parts.
4) Crankshaft, camshaft bearings float on a cushion of oil. Thus, thicker is a little better than thinner. Unless its cold and you are just starting up the engine.
5) Lower weight = better fuel economy.
6) Important! The wider the spread in cold/hot weights for a grade of oil, the worse the compromise made in formulating the oil. Ergo, generally the lower quality of the oil itself for durability and long mileage. 10-30W oil worked great on 70's engines. 10-40W not so good.
7) Synthetic polymers are more durable, slippery, and heat resistant than natural polymers. But all detergent oil picks up dirt (One of oil's primary functions) at the same rate. Eventually, any oil will become saturated, thus dropping grit out of suspension.
As you can see, it is not easy to figure out which compromises one wants to make, when selecting the oil to run.
