Well-enginered semi-synthetics are great for those that want the high-temperature stability and wide viscosity range of a 100% synthetic, but not the high-cost when doing a short drain interval (< 7,000-miles).

I use Schaffer's 7000 15w-50 semi-synthetic since it has a high TBN and excellent shear qualities due to the moly additives. It also meets Daimler-Chrysler's MB 228.3 and 228.5 specs which is fine for my C36 AMG engine.

Check it out here:

http://www.schaefferoil.com/datapdf/700.pdf

More info can be found on oils here, including the Schaeffer brand:

http://theoildrop.server101.com/cgi/ultimatebb.cgi

:-) neil