Also gauge only lets you know about SUDDEN oil pressure loss if you are checking it at the time. A LIGHT gets attention because of it's very nature. Also people tend to 'check the dash' only when they are stationary - oil pressure at it's lowest.
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