I always thought M103s/m104s liked thicker oil. My cert. Mbz mech always used 15W40, and then I tried out 10W40 for a while and just got better fuel economy. It ran slightly smoother though with the 15.

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1975 W116 280S - SOLD
1994 W124 E320 Coupe - Gone

CURRENT - 1974 450SEL - 1987 560SEL