Since you just bought it I would check the inline fuel filter and the spin on..they are easy to change out and if you don't know about the PO and how well he took care of it is something I would do right away. I run rotella 15/40 in mine but I live in the south where cold weather doesn't stick around long. Do a search on "oil types" or something like that and you can find what different people are using. From what I have read, synthetic is the way to go. My next oil change I will be changing to that. If you have a leak though, I would fix the leak before you switch over...sythentic can start to cost you some $$$ if you have a leak.

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1979 240d 216k (W123) Daily Driver
Volvo 245DL 210k

1979 300D FOR SALE
2002 Tahoe 91k FOR SALE

1980 300sd 177k (W116) Sold
1980 300sd Parts (W116) Junked
1979 240D 121k (W123) Sold
1982 300TDT 298K (W123) (Sold)