The following is a general rule-of-thumb for
NON-SYNTH oil and engine life:
Average Temp Range Average Engine Life
180 F to 220 F degrees 300,000+ miles
220+ F to 240 F degrees 200,000+ miles
240+ F to 255 F degrees 100,000+ miles
255+ F to 270 F degrees life <= 100,000 miles
Again this is for non-synthetic based oils. At 180 to 220 F degrees the non-synth oil provides optimal hydrodynamic lubrication meaning the moving parts will glide or float on each other without touching their metallic surfaces to each other. Above 220 F degrees hydrodynamic lubrication begins to "thin-out" and parts are only gliding on each other part of the time due to a drop in viscosity that is in an inverse-proportion to the temperature increase.
Synthetic-based oils are made to retain their max viscosity at higher temperature ranges before hydrodynamic lubrication begins to lessen. What the exact average numbers for SYNTHS are I don't know but I do know it averages around a 25% additional life in mileage. So given this 25% gain teh engines using
SYNTHS would average the following:
Average Temp Range Average Engine Life
180 F to 220 F degrees 375,000+ miles
220+ F to 240 F degrees 250,000+ miles
240+ F to 255 F degrees 125,000+ miles
255+ F to 270 F degrees life <= 125,000 miles
This sounds about right as Mobil 1 SYNTH claims engine life up to 400,000 miles. I would expect fully ester-based oils would have the same averages.
Hope this helps those folks who are worrying about the oil temps.
