Currently running 10W40 in my stroker (recommended by engine builder for clearances).

Thinking about switching to Mobil1.

I've heard Mobil 1 is slightly thinner than their advertised weight. Any truth to this?

If this is the case, can I run Mobil 1 20W50 in my car?

 

NO it is NOT thinner in this sense. It has less internal friction and so it flows easier. No do not use 20W50 unless the engine was clearence for it, set up loose like for drag racing. In fact with synthetics you can run a slightly thinner oil and get better protection.

 

Which brand of 10w40 are you running now? Synthetic? What did the engine builder reccommend?

 

Actually, different brands of "10W-40" oil can have significantly different viscosities. The SAE criteria for oil viscosity classification allow a fairly wide range of viscosities for any given "weight" rating. One oil might be one centistoke thicker than a "30" weight oil, and be classifed as a "40" weight, another may be one centistoke thinner than a "50" weight oil and be classified as a "40". "Patman" did enough testing of the xW-30 Mobil 1 to find out that is was measureably thinner (and gave poorer used oill analysis results) than the xW-30 German Castrol that gave him the best results when measured by used oil analysis. But that result can not necessarily be carried over to other Mobil 1 products.

Consult your engine builder. Mine specified not only oil weight, but also oil brand, based on his experience with that oil, and knowing that when he put the engine together. In any case, I would not use the 20W-50 for street use, particularly in coder weather, unless the engine builder recommended it.

 

20w50 valvoline vr1 race oil as per chad golen bearing clearances 25-30 thousandths

 

This greatly depends on your definition of "significant". I don't think a petroleum chemist or engineer would use this adjective to discribe the differences between different companies products. This rating is controlled by federal regulations, SAE standards, and the companies themselves. Every batch of refined oil must go through stringent testing prior to releast to the public. So unless a compny is delibrately ignoring their testing, it is unlikely that there would be significant differences between one companies 10W and another companies 10W.
And as far as Patman, I don't think he knows anything about oil!!! It takes a great deal more then one person with one engine and a handful of oil analyses to make a determination whether any oil is better then any other oil. Oil testing facilities run literally hundereds of engine in numerous test configs. over thousands of hours. It take these and the hundreds of thousands of oil analisist from the field before a company can even begin to determine if a change in, say an additive, is making a difference, or if their oil is holding up better then someone elses. We know absolutely nothing about Mr. Patman's "test", if you could even call it that.
Knowing a good deal about Mobil oil, I'm a huge fan. But I would never tell anyone that it's the "best". Only that it's one of the best.

 

Currently running Quaker State regular oil.

Wanting to switch to Mobil1 synthetic. Just wondering what weight to get.

 

If your engine builder told you to use a Xw40 motor oil and you want to try Mobil 1, then I would suggest trying either the 0w40 or the 5w40 truck and SUV oil...

Given that you live in a cold climate, these look like a good choice in the Mobil 1 family for what your engine builder reccomended.

That being said, there are lots of good oils... Mobil 1 is certainly one of them...

 

I'll leave the definition of "significant" up to the philosophers. Bill Clinton is still working on the definition of "is".

The SAE standards for motor oils:

SAE 30: 9.30 - 12.49 centiStokes
SAE 40: 12.50 - 16.29 centiStokes
SAE 50: 16.30 - 21.89 centiStokes

A motor oil of 12.49cS is SAE 30. A motor oil of 12.50cS is SAE 40. A motor oil with a viscosity of 16.29cS is SAE 40. A difference of 30% in kinematic viscosity is possible, for two oils carrying the same SAE "weight" rating. Is that significant? You decide.

My point was (and still is) that the viscosity of an SAE "weight" oil is not a single, absolute number. Is is possible for two SAE 30 oils to differ by as much as 30% in kinematic viscosity.