Originally Posted By: Panzerman
15w40 is made for mixed fleets which are gas and diesel. Use it, will see it is perfectly fine, summer or winter. I lived in Pennsylvania for 30+ years and we used 15w40 oil in everything, well below zero degrees without any problems or weird noises. Most people that reccomend aginst it have no experience to back thier opinion, other than what they believe "may" happen..
Oh, contraire, Herr Panzermann. I had a diesel tractor that WOULD NOT spin over quickly enough to start on 15W40 but would (and does) on 10W30.
Can you "use" 15W40 in engines spec'ed for 5W20? Sure. Is it optimal? No. Could it be harmful? Possibly, especially when viewed long term. Go to Widman's site and graph 15W40 vs a 5W20, view the results in context with the most likely range of climatic conditions. Unless a person lives in a very hot climate or uses the truck as a "maximum duty" workhorse, everyday in the summer, the oil will be running in the 50 grade. In winter it could be running as a 60 grade... or off the chart. Not sure I even want to speculate on cold weather flow (or lack thereof) in 20 and 30F temps. Typical 5W20 CCS is around 5300 cP @ -30 for a syn blend. A typical dino 15W40 is around 6500-7000 @ -20. Big diff in cold flow. You can narrow the gap with a syn 15W40, but not by all that much.
I would also mention that anyone that doesn't have a similar truck with a 4.2L V6 and has used 15W40 in it, especially thru winter, then they too are merely expressing an opinion on what "may" happen with the OP's truck.
To the OP: If you must use this oil in your truck, then the best time to do so is in the summer. Since the 4.2 has architecture based on the Windsor V8 engines, of all the newer Ford engines, you have the best chance of "getting away" with a heavy oil like that. But again, it's not optimal from the cold flow (and wear) department, as well as fuel economy. It's likely a two grade increase in viscosity is likely to show up in the MPG department, especially if you don't use it for long drives. The best way to make the most of an over-spec, heavy oil is to use it on long hauls, where the oil stays hot and relatively thin. IMO, lots of cold starts and short hops would be detrimental.