Manual transmission oil

[Triumph Man]

Hi guys, one of my main problems with my E34 520i (M50) at the moment is a whining gearbox. It is a manual, and uses ATF as stated by the factory. It has had a recent change with ATF and still whines. I know it might (read probably) be too far gone, but as a last ditch attempt before replacement, I was going to try manual transmission fluid. The theory being this is thicker and may quieten it down.

Couple of questions (and my flame suit is firmly on!) is what gear oil would you recommend for a manual 520i, and secondly, what issues may I encounter? Harder shifts I can live with, but I don't want the box exploding on me...

Eventually I will replace the gearbox and the clutch, but just wanted to see if I could quieten it down in the interim as I do not know when it will be replaced.

[roofer]

I'd get the thinnest available. Doubt it will shut it down though.