Originally posted by PeterM: Engine mfg's and oil Mfg's work together on oil specs. 5W-30 is specified for reasons related to fuel economy, valve train lubrication design (e.g see small oil channels), start-up lubrication performance, over all engine cooling.
Personally, having previously worked for Pennzoil and taken their lubrication training in Dallas, I would never go to a 15W-anything unless I had a deisel or the mfg spec'd it. 5W-50 Synthetic would Ok for extreme high temp operating environments. 0W-any thing would only make sense for seasonal application in extreme cold environments (like the arctic).
Hope this helps.
Amen to that!
And for all those to whom it applies, 15w weights are also used in many marine applications.
