It sounds like Automatic Transmission Fluid. Diff fluid should be clear and maybe a bit grey to black if its old.

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'12 E92 M3 ZCP Stage 2 Tune, Exhaust, Test pipes, GTS DCT Software
'07 E70 4.8i
'07 E92 335i FBO+Meth M3 Suspension Swap and much more 505hp 564 lb/ft tq (Sold)
'89 E30 325i (Retired with 230,000 miles )