96-99 E300D is bullet proof

I am sure the mechanic told you oil wrong - you should use 15-40, not 20-50; former is specifically designed for diesel engine with strong cleansing agents, the latter is for gas engine. The oil must meet MB 228.3 or up. Some people use synthetic oil but the car was designed using dino oil.

Additionally, you should change brake fluid every 30kmi, so does transmission fluid. Changing brake pads only when they are worn out as indicated on cluster panel.

I live in San Diego area.