I changed the engine oil, oil filter and air filter in our Mitsubishi Space Runner J reg (1992) 1.8glxi. It always had mineral oil in it but I used Semi Synthetic this time on advice from expert. Within 4 hours of driving, the engine is smoking, and the water pump is gone (seems like water pump) and it empties the radiator withi 30 minutes. There is also whining noise coming from the engine area. What did I do wrong. I know it has 100K on the clock and synthetic was out of question but semi synthetic in lieu of original mineral can not make that much difference. Also, why has water pump gone? That has nothing to do with the oil change. There is a tiny mayonaise in oil filler cap but that was there before too and is due to the seal inside the cap being really bad and not due to head gasket. But the head gasket may now have gone as the engine was on with the temperature gauge in red area for a few minutes. Can anyone tell me if it is easy to change the water pump (guess timing belt has to come off which will be a pain).