Any opinions on what's best? Just bought boat w Yamaha 90 4 stroke...

Yahmalube with Stabil!

Yamalube Ringfree stabil.

I use Yamaha Fuel Conditioner / Stabilizer Plus, and Yamaha Ring Free. They are two different things, serving two different purposes despite widespread misunderstanding to the contrary. Look them up on the Yamaha website.

I've been using these two products for many years in my outboards, mowers, chainsaws, yard equipment etc. I have used treated ethanol gas more than a year old with no problems.

If you Google Sta Bil, you will see a number of complaints about it thickening up, separating, and turning to gum. My neighbor used it in his garden tractor and was not pleased.

I will continue to use the Yamaha products as I cannot argue with success. Bombardier makes similar products (I think exactly the same ingredients) as the Yamaha stuff, but it's much easier for me to find the Yamaha brand locally.

When my 250 SHO was rebuilt the mechanic told me no need for ring free. Only on 2 stroke engines. Just use the yahmalube product as an additive.

Thanks for the feedback. Much appreciated.