I bought a pre-owned Ford Ranger from a used car dealer. I changed the oil three times (every 3K mi.approx.) using 10W-30 weight before noticing the owner manual recommendation to use 5W-30. I don’t know whether the previous owner used 5W-30 or 10W-30. Harm done? Which weight should I use now?
No harm done.
Next time you change oil use 5W30, as the manual recommends.
Almost certainly no big deal - but it depends a little on climate. IF you live in a place that gets really cold a lot - like below zero then I’d be sure to run the 5-30 in the winter. But if your climate is somewhat mild and rarely all that cold the engine probably won’t notice.