I know oil is like religion and politics. Everyone has an opinion...
I know several people who are running a Mobil 1 synthetic 0W40 in all their vehicles. This is a European spec oil which I understand are made to better standards than most API oils. People are running this in engines that call for 10W30, 5W30, 10W40, etc. and just buy this quality oil in bulk at a low price. I wouldn't be worried about the 0W in the spec for cold weather starts but was wondering if a 40W oil is too thick for driving down the road with the engine warmed up. Is this a bad idea? I know you might take a small hit in mileage which is one of the reasons everyone has started using thinner oils but would it be harmful or helpful with regards to engine wear and operation. The vehicles in question are a 2000 S-10 with the 4.3L and a 1997 Ford with the 4.6L The 4.6L is the 2 valve model and I don't think there are any issues with cam timing, etc. on these. I know some of the newer models used cam phasers and needed exactly the right oil to operate correctly. Both of the engines I mention call for 5W30.
I know that running an oil slightly out of spec would be better in a pinch than running it low but I am curious as to what opinions are here about this. I also know people who run the Rotella T6 5W40 synthetic "fleet" oil in a variety of vehicles and applications without issues. This is what I run in all my outdoor lawn equipment as it isn't much different in price than a quality conventional oil.