Have any of you tried running 15w40 through gasoline engines? I was given a few gallons of Mobil Delvac and told that it is good to run in the summer heat. I haven't really tried it though because I don't wanna screw anything up. I know that it's HUGE in the diesel industry, but there's gotta be a reason my manufacturer recommends 5w30. I just wanted to see if anyone on here has seen any negative or positive results from using the diesel grade oil on a gasoline engine.
It doesn't help. Funny enough, oil temps don't fluctuate that much between winter and summer on a hot engine (not in a water-cooled engine, anyway). The big difference is cold start. You can get away with a slightly thicker oil in the summer because the oil won't be as thick as syrup when you start it in the morning, but 5W30 is still going to be better cold and almost no thinner hot.
If you want the best of both, 0W40.