I like Lucas oil additive. Its like the old STP oil treatment. I also use Rotella T 10W30 by shell in the cuda. Its a heavier duty oil.
Personally I also take fish oil, 2 pills a day. Fish oil is an EFA (essential fatty acid) that increases your HDLs, the good lippo protiens that carry cholesterol out of your blood back into the liver. Its also a joint anti-inflammatory. Maybe putting a few pills in the old 318 would do it some good, eh??