Use the service book schedue but divide by 2 for oil changes (ie twice as oftern). Run the highest octane available to you. I run all 5 of my BM's on 98 octane and they love it. Drive it lots and love it to deatth..
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i