Mine sometimes just goes straight in the wax unless it’s filthy, putting water into the oil at 190C would be a very bad idea so washing in white spirit/paraffin if nothing else makes sure it’s water-free. And it only takes a minute as the chains off anyway.
There’s a bit of grit in the bottom the tin (now in the fryer) but it’s never come out stuck to the chain.
Not sure about the used engine oil idea, I found if I tried to top it up with oil it just turned to a viscous mixture on the chain which eventually washed off. Otherwise it’s not an issue though IME, I’d probably re-apply it as necessary (every couple of months on average unless I was going away for the weekend and was giving the bike a once over or doing a lot of miles on one bike). I think putoline recommend topping it up with their spray wax but I’ve not tried that.
Part of the benefit is not having to top it up anyway. Not being able to top it up after a ride is definitely compensated for by not having to top it up for 2 months.