With 6 to use they all get a fair go. The R56 is on a miles related lease so has to be used most. I think we tend to agree on most oil issues; as much depends on usage as anything else. Mine is 80% urban in rush hour - the rest play.
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