I recently had a clutch change and as a result the gearbox oil was changed, almost certainly to a differrent type. Castrol Syntrax 75W/90 oil is in the gearbox now, but I don't know what was in there before that. Since the oil change I have noticed a very faint whine from the gearbox. At the moment I'm thinking this is because of the different oil not working as well and want to try a differrent oil. Can anyone recommend an alternative, perhaps slightly thicker, oil?