If you let stationary engine and antique tractor collectors join the competition, the mower guys won't get a look-in. Some of the experienced tractor guys walk up to old tractors that have been out in the paddock abandoned for more than 50 years, get them started in 10 minutes to an hour, and drive them onto a trailer. The trick is to let the water out of the bottom of the sump without letting the oil out. You probably want to let it warm up a bit on that old oil then drain it out, so it will take some crud with it. That way you can run it for longer on that first lot of new oil (not long, but longer).
Warning: do not do this kind of thing with old racing engines that may have run with castor oil as lubricant. Major disaster may result. I recall somebody who I won't name destroying the engine of a borrowed Maserati 300S that way. The castor oil leaves a varnish on the crankshaft bearings and in the cylinder bores. When the engine warms up sufficiently the varnish partly melts and if it is mixed with mineral oil it acts like instant superglue. If you are in the middle of doing a few demo laps on race day at Sandown at the time, it can end badly.
BTW, I agree with you that if the used car-engine oil is still clear, it should be good enough for the mower so long as you change it regularly in the mower. The main enemy is suspended stuff in the oil, and that pretty much always affects its clarity. However clear is not the same as 'I can still see some daylight through it if I hold it up to the sun'.
