I do used oil a lot - I change my motorcycle oil (non gearbox) at 6,000km, keep it and use it to top up my daughters oil burner when she shows up, she never checks the oil, so it's always low.
My mower had used oil too, and it's really old. I put some synthetic oil of unknown viscosity into a bike late last century, and only ran it a few times. A couple of years ago I rebuilt the bike, and changed the oil of course before running it. That oil has been in the mower for the last couple of years, still nice and clean. This weekend I might change it for some 5W30, and will do a temp test before and after like Shannow has done. Free mowers are good to experiment on - if this one blows up I'll just give it back to the SIL and say, ''It's really stuffed now mate, you better get rid of it.''