My 89 Carlsson has been religiously serviced but i note that the oil level in the diff is well above 'max.' Horrors, there is no drain plug! Haynes says there is but where there should be there isn't. When I got the car at 151k there was no whine at all, but the rubber bush in the gear selection linkage was almost totally u/s (gear stick all over the place). When i fitted new bush, noise appeared at once. I want to drain and refill. Should i?