These engines do not have roller cam followers as do modern engines. For that reason, your standard Premium engine oils do not have a zinc additive that is required for long cam and lifter life. The oils that still have this are oils such as Chevron Delo, Mobil Delvac and Shell Rotella T. These oils are primarily designed for diesel engines but work out quite well in the M103. I have used Chevron Delo and my 300E now has 320,000 miles and is still in top shape.