As we all know, oil shears, resulting in a lower grade of oil (a 30w becomes a 20w) after a few hours. with this in mind, would it be a good idea to switch to a XW-40w oil? I figure after a little while, you would be down to a 30w, which is what the mfgr. recomends. do any of you run 15w-40 in your engines? do you have engines with over a 1000 hours that have only been run with 15-w40? have you had any problems with the engine that could be trace back to oil problems? is there any reason I should not run a 15w-40 oil in my engines? thanks.