I used to have my
oil chanded in my work car but when I bought a new 2001 F150 that requires 5W-20 weight
oil that all changed. Unless I want to make an appointment at the local Ford dealer I cannot find any place that carries that weight
oil. When I asked every one of the quick change places said they just put 10W-30 or 10W-40 in and it would be OK. I said what about what the manufacturer recommendations. They all say it is just for gas milage. I say that may be true but if I have an engine failure for using heaver weight oil it could void my warranty.
I think the new engines are built with closer tollerances and therefore require thinner oil. Kind of like the old high pressure/volume oil pumps causing distrubutor gear failures. If it was ok to run 10W-30 0r 40 then it would be listed in the manual.