A mate of mine decided to change the oil on his Triumph Dolomite. He added some engine cleaner stuff into the oil and ran it for a time as stated in the instructions then went under it and undid the sump plug emptied the contents then did it back up. After that he added the quantity of oil it stated on the engine. Once done he came to the pub proud of his first foray into mechanical self reliance.
A few days later he mentioned that the gears were crunching so we all piled round after the pub. One of us checked the dip stick and found that the oil was 3/4 up the stick. We then worked out what he had done.....
He had only emptied the gearbox! Then poured the new oil into the engine with the old oil and the engine cleaner.
So new engine AND gearbox oil later and no damage despite 100+ miles.