Bob - The best motor oil for older engines is HD diesel engine oil. It has a better additive package than any modern spark ignition oil regardless of base stock. There is much discussion about this in the archives.

Mercedes' recommended oil change interval for our cars is six months or 7500 miles, whichever occurs first. My recommendation is every 5000 miles or one year for cars that see less than 5000 miles annual mileage accumulation.

Duke