Originally Posted By: sfloyd33
Originally Posted By: Virtus_Probi
252F is only 122C, nothing to really worry about as a spike. NOACK is tested at 150C (302F) for an hour. Oil flashpoints are usually well over 200C (392F).
Were you towing up steep grades while running the AC?
I've hit over 230F in my little SUV with just my family and a load of softball gear in the Monadnocks in the mid 90s. Thicker oils will tend to run hotter, you could probably run a bit cooler with a HD 10w30 but the oil film thickness will still be greater with the 15W40 under the same conditions even if the latter oil is a bit hotter.
My oil temps stayed pretty steady in the 250 area for a long time under throttle. However when I was idling the temps came down to a lower level fairly quickly. Yes we were using the AC and running about 75 mph on the interstate through KY.
I'm not sure if it's current, but I found a PDS for Motorcraft 15W40 that claimed a 441F flashpoint (it paired that with 277C, must have been a typo as 441F=227C). I know you're not worried about your oil catching fire, but that number suggests to me that your oil can handle 250F for some time. I wasn't able to find NOACK loss for that oil, but that test is not a good direct comparison for engine use because the sample has a constant air flow over it to immediately carry away anything that vaporizes.
Did you happen to notice any unusual oil consumption due to your trip? If you did, maybe it would be worth looking at an oil that is especially robust at high temps. It not might be a good match for your application and tends to be expensive, but M1 5W30 ESP (Euro diesel oil) has a low NOACK loss of around 6% and a very high 254C (489F) flashpoint. Its 3.58 HTHS is very close to some XW40 oils, M1 0W40 FS is around 3.6.