oil grade 5w30 or 20w50

[Dave Volk]

I just bought a 98 740il and I want to add synthetic oil. Do I use the 5w30 or 20w50.

[rjp325i]

Why not use Castol Syntec which is available in 5W-50 formulation at each oil change.

[leonsjoy]

New to BMW, just bought a 1987 735i L7. Has a crack in the pan that I plan to fix(remove and have welded) hopefully Monday. Should I use a Synthetic oil in it since it has 176,000 miles? Like Castrol products. Thanks

[rjp325i]

It is kinda late to start using synthetic unless you are sure of all the seals, gaskets etc. Synthetic is very highly detergent and will dissolve old crud and possibly invite leakage. You may luck out with the seals but it will be a gamble. I would stick with a good quality conventional such as Castrol 20W-50 especially if if does not go below freezing too often. Castrol Synthetic blend might be an alternative but I have no experience with it. My 1989 325i (bought used) has been using synthetic since 36K miles and now has 125K with no oil consumption and oil changes at 8K mile intervals. My first use of synthetic was with my 1985 318i bought new. At 202K miles I sold it and it never required oil between 8K mile changes. At first I used Mobil 1 15W-50 but later switched to Castrol 5W-50. Both are good as well as Red Line synthetic products. Red Line 75-90 for the differential is great and can be used with limited slip.

[leonsjoy]

Thanks!
Suspected that but wasn't sure. Will stay with the conventional Castrol. Have used it in my other cars and have had good results. Those that I have opened up appear much cleaner than other brands.. Appreciate the advise. Thanks again

[rjp325i]

Glad to help. Good luck with the 7. Before I switched to synthetic I used Castrol 20W-50 year round back in Smithtown, NY (Long Island). Worked fine, no problems. It was used in my new at the time 1985 BMW 318i.