5-30 in any stovebolt is a HUGE mistake. These engines aren't designed for water-thin oil. I believe any oil with a lower viscosity number below 15 will tend to hammer out bearings and make the engine sound like there are dozens of little guys with hammers in there trying to break out.

I use 15W-40 in everything I own, from lawn mowers to farm tractors. No oil-related failures yet!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!