GF-4 Based Oil vs. GF-3 - What is the difference?

Mobil 1 5W-30 meets the standards for ILSAC GF-4 (API Certified - Starburst). I suspect that no change in the formula was necessary to meet the new standard (but don't know for sure).

Mobil Clean 7500 (synthetic blend) and Mobil Clean 5000 (conventional oil) also meet the new standard.

For some reason, Mobil 1 Extended Performance oil does not meet all of the ILSAC GF-4 standards. Could be that it does not meet the stricter fuel economy tests, or has too much phosphorus which can shorten the life of catalytic converters. It does meet the engine wear tests.

Basically all one needs for the current starburst standard (now GF-4) is to meet the current API standard along with its corresponding "Energy Conserving" standard.

I think you'll find as oils are relabelled for API SM, they'll nearly all meet the EC standard and thus GF-4. There might be a few exceptions, including some "European Formula" 0W-30 or 5W-30 oils that are designed to be on the heavy side of 30 weight, and won't meet the energy conserving standard. Pennzoil actually markets two different 5W-30 oils in their "Pennzoil Platinum" series.

You've got that one. They have extra "SuperSyn" which is what Mobil calls a "high viscosity index polyalphaolefin". Basically what it does is serve as a base oil that also increases the viscosity index. I've heard using it also eliminates the need for other VI improvers in weights that previously needed it (or they now need less VI improver). So what you end up with is a slightly thicker oil (closer to the max for the viscosity range) at operating temps, which reduces fuel economy.

ILSAC GF4 is only met by API latest standard energy conserving grades. There is a misapprehension that some European specification oils that meet higher performance standards will somehow meet and [easily exceed] the API standard but somehow fail the 'energy conserving' starburst. In fact it is probable that the only reason that the starburst is not awarded is that the sales volume of these oils would not warrant the accreditation and licensing fees levied. In Europe where these oils are commonplace the starburst and ILSAC standard is not used. Instead the owner is trusted to use appropriate quality and viscosity for his application and to be intelligent enough to know that a thin oil will be more fuel efficient. Effectively any oil with the required API rating and a 'w' rating of 5 or lower and a hot viscosity of 30 or lower would qualify for the starburst if a wad of money changed hands.

Huw