On a related subject... I spoke with a guy at Wearcheck, and he said that for gas engines they recommend getting TAN tested, but for diesel engines, they recommend TBN instead.
I guess getting both TAN and TBN would be best, but if you were only to get one, which one would you get for a gasoline engine? Most of the gas UOAs that I see here on BITOG have TBN, not TAN.