The Getrag 260\5 boxes use Gear oil & i think the 260\6 does too
My 89 E30 has the 260 and ive seen a few older E34 525i '88.'89 era that use them too. If it doesn't have a ATF label on it probably use SAE 80 gear oil
You know when you drain it so not really a problem.
Just put some MTL into mine, not as notchy, smoother and eliminated my little bit 1st to 2nd gear syncho grind.