Hullo folks, I'm looking at a T25 tomorrow, a 1.9 petrol. It has recently had an oil flush and oil change, using 10w 40 semi (now this isn't an oil type thread, have faith

). After this the engine developed what sounded like a tappet noise. The chap that owns it took it back in and asked them to drain and use heavier weight mineral, which they did, and the tapping has now gone.
My question is what would cause the tappet to play up using semi? Could it possibly have been caused by something loosened by the flush (which I've never liked doing myself)? I just want to be sure that the heavier weight oil hasn't been used by the garage to mask a problem.
Thanks for any help, it's much appreciated
