22-05-2012, 09:06 PM
I think I can probably dig out some info for you - I work for a company which developed a transformer oil quality tester - but I'm on my hols until Thursday. You might consider draining and doing a complete refill rather than top up as the stuff ages and becomes acidic. In fact, thinking about cost, it might be better to have low-spec new oil rather than (mainly) high-spec old oil?
