I doubt that the use of straight 30 weight oil will kill an air cooled engine in a year or less.
Probably a few other factors involved to cause such a short engine life.
I won't admit to do it but have been accused of running some unrecommended lubricants in engines and have never come to grief over it, yet.
Neighbor is running 5W20 synthetic in a 1970's vintage Kohler air cooled right now, as we speak. It's fairly warm out there right now, about 95 and he's cutting grass. Told him last year it I thought it was a little light but he just shrugged, said it would be a good excuse for a new zero turn mower if the engine blew.
If I can't fix it, it ain't broke.