transmission Preventive Maintenance for 1995 E320

[Maruzo]

Hi all. I've recently acquired a 95 e320 with 170,000 miles. Put on close to 15000 miles already in 12 months. I've heard these cars can go over 300,000 easily. I'm using it primarily for commuting to work and back.

Question:

What's the best preventive measure for the transmission and the gear box?

I've heard that if you replace the oil in the gear box/ transmission that would help delay the eventual need to rebuild the transmission.

Is there an easy way to do this? Or should I leave it to the mechanics?

Which oil should I replace? transmission or the gear box? And does anyone know how much a job like this typically cost?

Much obliged!

Theo