I put Mobil One engine oil, I don't remember what weight, in my '84 Euro 240D manual 4 speed at about 300,000 miles. I have yet to change it, and the car now has 550,000 miles and the transmission is still as good as new. The gear oil is too thick and will build a bit of heat in the summer, and be difficult to shift while cold in the winter.

Good luck,