Found this on another site from a very knowledgeable guy.
"The latest GF-5 Spec provides significant improvements over API/SM in a number of areas:
1) more durable friction modifiers, to provide better fuel efficiency for the entire service interval (note: only the 20wt & 30wt grades are friction modified)
2) Enhanced compatibility with fuels containing ethanol - up to and including "E85". (Ethanol in the crankcase combines with moisture & can form harmful emulsions)
3) Better Phosphorus retention, to optimize oxygen sensor and catalyst life
4) Better protection from high temp, piston and turbocharger deposits - there is a new test specifically for turbocharged engines
5) Improved protection from low temp sludging; this analogs short trip driving in cold weather, where the oil may never fully heat up.
Note that GF-5 is somewhat different from API/SN. It's applicable to domestic & Asian vehicles and is a more stringent specification."
"The latest GF-5 Spec provides significant improvements over API/SM in a number of areas:
1) more durable friction modifiers, to provide better fuel efficiency for the entire service interval (note: only the 20wt & 30wt grades are friction modified)
2) Enhanced compatibility with fuels containing ethanol - up to and including "E85". (Ethanol in the crankcase combines with moisture & can form harmful emulsions)
3) Better Phosphorus retention, to optimize oxygen sensor and catalyst life
4) Better protection from high temp, piston and turbocharger deposits - there is a new test specifically for turbocharged engines
5) Improved protection from low temp sludging; this analogs short trip driving in cold weather, where the oil may never fully heat up.
Note that GF-5 is somewhat different from API/SN. It's applicable to domestic & Asian vehicles and is a more stringent specification."
