I was going to post my results on two items - a 1987 B&S 7hp air cooled engne and my 91 BMW 318...
But Ill post the results for the Briggs now...
Dumped the oil and put in pennzoil longlife with 2/3oz neutra (1 1/4pt sump). Usually the mower stunk when driving around, but the emissions were not nearly as bad. I then drained that after mowing an acre of hilly lawn, and refilled with rotella, which I had around, plus some neutra in the gasoline. The mower had cooled to the touch, but it immediatelystarted, no need to crank it more than a slight fraction of a second, it caught immediately. ran smoother than before, and not as stinky.
I should add than usually the oil in the sump is black. With fresh oil and neutra in it, the oil was relatively clean, with black ctreaks that comprosed of nearly microscopic specs. So I think it was doing its job.
Hasnt burned oil since, we shall see if it smokes at 90+ degrees, thatll be the major test. One way or another, I am satisfied, and think it worked great. SInce it is an air cooled and thus hotter running engine, Id suggest doing it on a cooler day, and not leaving neutra oil mix in for too long, as I would have to guess that it will lower the oil viscosity, even if just a tiny bit.
JMH