what kind of engine is on that garden tractor, and how old? 15w50 is a little heavy for an air cooled small engine
if you have issue with gas dilution, check the valve lash (if 2 cylinder, check both sets) and also if you have the universal fuel pump
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which most small engines in the last 20 some years have used(That style, they can be metal, mounted differently, etc..) then you should replace it period and also replace the vacuum line that leads to the engine (the place the vacuum line goes depends on the engine, mine on my kawasaki goes to the crankcase in a special spot)
The pump diaphram may be allowing your crankcase/
intake/engine to suck gas into the oil. Or valves could be out of tolerance for lash and then excess fuel finding its way washing down or pushing past seals etc....
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Finally, Ive personally had good luck with a oil burner (small engine) simply from high hours and tender abuse (high loads lol) and using this stuff. According to their website, that 9 oz can(4 cylinder formula) about 4-5 oz in your small engine + oil and new filter(Dont overfill, you have to substitute a small bit of oil with that) is the best amount in a small engine. You can use it in a larger engine, car engine. I have personally used it although I have little to say about it since I was using it as preventative measure. The stuff is a sacrificial coating that lays down into the cylinder walls and then increases compression/sealing so you have more HP, less gas and blowby etc.....
Project farm used it on a diesel tractor and had fantastic results and the reduced oil use Ive seen has me convinced.
Do that, a valve job, and the fuel pump+vacuum pulse hose, I think you may see a reduction in your issues.
At the very least, you should extend its life a bit. Maybe run a bit better.