Looking at my manual for my '06, it says not to use 5w40 in temps over 90. Why not? In hot temps 5w40 should be same as 15w40. The difference would be in cold temps.

I guess they make those recommendations because , at high temperatures, some 5w40 oils really will not have as much viscosity as a 15w-40. However, many people at dieselplace use high quality 5w-40 year round.

I believe the caution is based on the Grade III "synthetics" that get their viscosity range from Viscosity Improvers (VIs) in the additive package to extend the range of the petroleum base oil, which is thin to reach the low temperature end of the range, this necessitates the high end protection provided by VIs that break down under heat. A true Grade IV or V PAO synthetic maintains viscosity across the entire range without the needed support of VIs and won't break down and thin out under heat.

A good high quality PAO syn 5w40 will be fine. The oil will not get too thin at all.

So the 5w40 rotella syn is one oil that should not be used above 90 deg due to the fact that it is a base stock 3 oil and not a true syn.

Whats the difference between an engine running 190 degrees in 70 degree air temps or an engine running 190 degrees in 90 degree air temps. :rolleyes: The oil sees whatever temps the thermostat dictates.......I never found the real significance of a lube chart for water cooled engines. Air cooled (non regulated temp) would be different.

One point i can think of now is the turbo, with the outside temps warmer even though it is water cooled the heat in that area will be warmer or generate higher temperatures. when i owned a dodge my oil temp in the summer would or could run up around 230 when the engine was working towing. now in the cooler weather the oil temp would run 205 ish. just a thought.

One point i can think of now is the turbo, with the outside temps warmer even though it is water cooled the heat in that area will be warmer or generate higher temperatures. when i owned a dodge my oil temp in the summer would or could run up around 230 when the engine was working towing. now in the cooler weather the oil temp would run 205 ish. just a thought.

IMO a 20-30 degree inlet air temp change shouldn't cause the turbo to significantly contribute to higher oil temps.

I do agree a marginal cooling system would benefit from cooler air. (once the stats are full open)

Getting back on topic though a 5w40 verses a 15w40 should be no different once normal operating temp is reached.

I guess what i was trying to say before is. The cooling system on these trucks can handle a certain number of btu's. Past that the engine will overheat. The warmer the weather is, The warmer the engine and engine oil and engine coolant will run. If the 5w40 base stock three oil can not handle higher temps due to being a base stock three oil I.E. needing viscosity improvers, Then the oil will break down and fail at a lower temperature. The load placed on the engine will directly effect how warm the engine will operate at,just like the outside air temperature will directly effect how warm the engine will operate at. Not trying to start a oil war but a base stock three oil is not a true syn oil as far as i am concerned.

I guess what i was trying to say before is. The cooling system on these trucks can handle a certain number of btu's. Past that the engine will overheat. The warmer the weather is, The warmer the engine and engine oil and engine coolant will run. If the 5w40 base stock three oil can not handle higher temps due to being a base stock three oil I.E. needing viscosity improvers, Then the oil will break down and fail at a lower temperature. The load placed on the engine will directly effect how warm the engine will operate at,just like the outside air temperature will directly effect how warm the engine will operate at. Not trying to start a oil war but a base stock three oil is not a true syn oil as far as i am concerned.

:agreed:

What I can't understand is why GM would say a 5w40 would be worst at normal or higher operating temps than 15w40. Assuming all 5w40 oils are synthetic (or wanna be synthetics) and most 15w40 oils are dino. :confuzeld Aren't they all 40 wt @ 100 c with synthetic having the edge at higher temps.

Rotella was involved in a law suit regarding rotella 5w40 syn oil. They use a base stock 3 oil and call it a syn oil. It is my understanding in order to get the 5w40 spread using a base stock 3 oil one of the additives they have to add is viscosity index modifiers. In doing this as the oil is heated during use in the engine the viscosity modifiers begin to break down, meaning the oil becomes less than a true 40 wt oil. Other true syn oils use base stock 4 and 5 oils, meaning they can obtain the 5w40 spread without using viscosity modifiers. I am not saying rotella is a bad oil. GM probably made the statement to cover the oils out on the market that use base sock 3 oils knowing the base stock 3 oils can break down with time and heat. I used delvac 5w40 in the dodge and it did very well in temps from 30+ below zero to over 100+ deg F. If all oils badged as syn were true syn oils it would be a different situation, however it is not so with the oils we have. True syn oils are available and are excellent at doing what they claim. sorry for the long post.

Any good synthetic is ok

I can say that Mobil turbo diesel truck holds the best oil pressure in my truck in hot temps and towing. Better than any other synthetic I've tried both 5w-40 and 15w-40 and better than dino 15w-40's. I'm sticking with it.

I believe the caution is based on the Grade III "synthetics" that get their viscosity range from Viscosity Improvers (VIs) in the additive package to extend the range of the petroleum base oil, which is thin to reach the low temperature end of the range, this necessitates the high end protection provided by VIs that break down under heat. A true Grade IV or V PAO synthetic maintains viscosity across the entire range without the needed support of VIs and won't break down and thin out under heat.

So I'm assuming that Shells Rotella T synthetic SAE 5W-40 is a wannabe Grade III synthetic and Mobil 1 Turbo Diesel is a full grade V synthetic?

Yes, Shell Rotella T "synthetic" is really a group III mineral based oil while Mobil 1 Tubo and Mobil Delvac 1 are both Group IV for real synthetic base stock oils. And yes, the Delvac 1 5W-40 will provide vastly superior hot weather performance vs. a 15W-40 mineral based oil. The base stock used to make a 15W-40 mineral based oil is a 15 Weight oil. The base stock used to make Delvac 1 is a 40 weight PAO base stock. You be the judge.. Which base stock will provide better high temperature protection; a chemically/plastomer modified 15 weight oil or a true 40 weight oil?
Not rocket science here, just good common sense..
Just because the Delvac 1 synthetic 40 weight base stock happens to flow to 60 below zero and gets a 5W (winter) flow rating, it is still a much thicker operational temperature oil than a comparable mineral based oil...
Do not be deceived by the "W" winter rating....
George Morrison, STLE CLS