Good afternoon fellow oilers:
Other than the weight, (and price), what is the difference between 20W50 and 15W40?
Thanks for the imput!

AFAIK,
15w40 is primarily marketed/formualted for HD diesel engines, while 20w50 is aimed at the gas-powered crowd.

A 15W40 is going to have a much better aditive package and will more then likely be less volitile! 15W40's usualy are shear stable were most 20W50 shear badly some down to a 30Wt.! If you want a good 15W40 Delo 400 and Delvac 1300 are the easiest to find and have excellent formulas. If you can find them in their CI+4 form they are even better! 15W40 has better performance in it's dino form at both hot and cold extremes then most dino 20W50's!

Bear in mind that, although I am a big fan of the all-fleet oils (Delo, Rotella, Delvac, etc), they are generally not recommended for use in strictly gasoline-powered fleets because they have a greater tendency to foul spark plugs. I believe this is due to their much higher DD additive levels. Most 20W50 oils would not be susceptible to this issue. In my experience, most 15W40s will not cause troubles either, but if you start fouling plugs more quickly/frequently, go back to a 20W50.

For my part, I have not experienced 20W50 shear-down.