I have always used the Halfords 20/50 Classic oil and it seems fine. But agree with Richard’s comment re sludge. A well-wisher sent me a magnet to add to my drain plug. It’s amazing how much ferrous stuff it catches but can’t catch the other rubbish. I assume the magnet does most of its work when the engine is stopped – especially when hot and the particles just settle in its direction. Do you know the history of the engine?
Chris
Chris