Varnish with Synthetic Oil question

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Cook the oil, you mean accelerated thermal breakdown due to running the oil warmer than normal operation? I'd put that with track use and towing.
I guess I'm thinking more of my personal example, my 84 Cutlass has an aftermarket aluminum rad and in extreme heat when stuck in traffic (like waiting to cross the border coming back from Syracuse nationals car show), temps crept up to 220+ before I turned off the AC and started bringing up rpm to cool it down.

Or about 5-6 years ago the cooling system got really dirty and needed a flush. Before flushing it would get to 230 on the highway, which actually destroyed the starter before I got to the flush.

Or when I had electric fans on a toggle switch and forgot about them until hitting 260 climbing a steep hill in a campgrounds. (eventually switched back to a stock clutch fan - hence the shroud issue now in hot weather).

Not exactly problems your typical trade every 10 years consumer would have but if you drive a "hooptie" for multiple decades these things can happen.

So, yes you're correct, if everything remains in perfectly ideal conditions, no towing, abuse or other mechanical factors at play, conventional oil will hold up fine.
 
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oil temps tend to track more with rpm than with coolant temps if both circuits are seperated. Newer cars have interchangers so the temps are closer together. But even 260F isn't dangerous for the oil yet if it's thick enough It's getting there for the coolant though depending on concentration and pressure
 
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oil temps tend to track more with rpm than with coolant temps if both circuits are seperated. Newer cars have interchangers so the temps are closer together. But even 260F isn't dangerous for the oil yet if it's thick enough It's getting there for the coolant though depending on concentration and pressure
I would add to this that in modern engines the coolant systems are more capable of handling temperatures due to fans etc....where as oil capacities are lower, and oil coolers (if any) are normally passively radiated, so not as efficient as keeping oil temperatures in check. Synthetic oils are much more capable of handling higher temperatures.
 
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