The last time I changed my gearbox and diff oil was in 2021; at that time, I put Castrol Syntrans 75w-90 GL4 in the diff. So I’ve probably done no more than 3k since then but when I drained the oil out of the diff today the oil had a lot of very fine brass specs in it. I’ve used various gearbox and diff oils over the years but never has there been any brass specs like today. The diff looks and sounds fine but I’m wondering how long it will remain like that?
The only brass I can think of in a diff is shims, potentially for side play but they dont usually disintegrate, especially with regular oil changes. I would drive it and change again in a year unless you hear any whines from the diff. Have you a magnetic sump plug in there to make sure any metal filings are removed, incase this is also causing an issue.
Is there anything in the diff with a protective coating that could be brass coloured?
I’m asking as my other car had a replacement radiator fitted, now my coolant has brass coloured flecks in it but I’m told it’s just the coating breaking down, not a problem - so I wondered if a diff could have a similar coating somewhere.
Is this your MK1?
Shouldn’t it be GL5?
Castrol site recommends Castrol Transmax Limited Slip 75W-90 for the diff on a Mk1 which is GL5. Funny thing is, I thought GL5 was brass destructive compared to GL4.
Brass in coolant ? You dont mention the car. Old cars have rad ends brazed with flux and brass mix rods is probably the only thing I can think of on that one.
GL4 and GL5 shouldnt make a difference on brass destruction.
Found this but not researched it any further , as sadly at work… yes … on here as well 