Well, Synethetics are totally different than conventionals. A synthetic with flow through a motor 10 times faster than it's conventional counterpart. You can run whatever grade you like since a synthetic has higher boiling and burnoff points and lower pour points. A 50 weight oil will have more parasitic drag than a 40 or 30 weight oil due to the "Weight" Difference, so there will be a hp loss compared to running a lighter grade oil, which is also a reason why 5w30 and 10w30 are rated at "Energy Conserving" and recommended for almost all vehicles since it has less drag on a motor compared to a 40 or 50 weight. Higher weights take higher temps, but not regarded when using synthetics, again because of their high boiling/burn off points and low freezing points compared to conventional oils. A heavier oil will provide more "Cushion" compared to a lighter weight oil. Also, the difference between a 5w30, 10w30 will be the flow charactaristics below 100 degrees. A 5w-30 will flow through a motor faster than a 10w30 until operating temperatures reach 100 degrees. A 10w30 will outlast a 5w30 due to having to use more polymers to cover the range difference between the two, leaving less room for friction modifiers and seal conditioners within the additive package. Running different grade oils, especially synthetics, shouldn't cause any mechanical problems, be it seals, rod knock, or any other failure due to oil starvation or oil pressure, if it happens, it would more than likely have happened with any other oil. Again, comparing synthetics and conventionals are a totally different world. Almost all ratings and recommendations are made for conventional oils.
Just from observation, but the Mobile 15w50 synthetic is as thick as dino 10w 30. It's strange, but that's what it looks like to me as I've seen dino 15w50, and you can actually tell it's thicker than the 10w stuff.
Just move it around in the bottle, and you can feel/see it.
But, I'd just run 15w 50 all year round.... after you start it up and race hard, I'd rather have something stiff no matter how cold it is outside my engine.
No it does not. Ever. Not unless it is run outside of its temperature range.
Multiple viscosity oil is always thinner when cold. That's the whole point of it. Single viscosity oil (e.g. 50w race oil) will always have the same viscosity in its operational temperature window.
The only way cold oil will make more pressure is by using a single viscosity oil (e.g. 50w) in very cold conditions, which is retarded.
15w50 is 15w when cold. There's no way in hell it's going to make more pressure at 15w then it will when hot at 50w. Period.
The 2jz crank seal issue is not related to thick oil.
The conclusion made about BMW's start up sequence with the increasing rev limit is also false. It has jack to do with oil pressure, and everything to do with preventing wear, specifically on the piston rings and cylinder bores.