Any 'Regular' = DEATH to BMW engines. Here in Australia we are lucky and get 98octane as the top fuel product (BP/Shell/Caltex/Gull). I never buy anything else - even for my mower! Steer very clear of Regular and you will do fine. Good luck with your oldie!
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i