OK bought a used lawn mower with a B&S engine. est 1995 but doesn’t really matter other than it runs well.
In a pinch, I’ve used 50:1 gas in the mower. Runs OK. I remember my Homelite saw use to run 16:1 and poorly. My 30 year old shindawa trimmer and saw ran at 32:1 and my Echo saw I run at 50:1.
Question: I can’t think of any reason not to run 50:1 gas in the B&S mower. And I prefer to keep one tank of gas rather than two tanks of gas which means I can use the gas quicker rather than let both get stale.