One of my machines has 1322hrs on the GX390 and it has performed flawlessly!!! Besides the normal air/fuel filter changes along with scheduled oil changes I haven't even needed to adjust the carburetor and even with the above hrs on the engine, It will still go the 45 hr oil change interval without needing to add oil...
1322 hours she is finished breaking in, last big block briggs I replaced at close to 4000 hours, it was getting old and I wanted something more efficient. 250 hour synthetic oil changes, 2 outer air filters, one inner, a couple fuel filters, and 3 sets of spark plugs. Only issue I had was the fuel pump, put a 12v 5 psi pump connected to key on and she ran great. Since I was the only one that ran the engine I always had spare plugs, fuel filter, and even an aux pump. Engine never left me hanging, dealing with gas and having to refuel the girl though was what did her in. Premium fuel keeps her going, I would not worry about an additive with prem fuel. First 2/3 of the engines life it got regular gas, found the premium just had the engine running better the last 1/3 and it was cheaper then replacing it.
Always keep in mind, we make our money running the engines, not fixing or replacing them.