You could get about 1,000 different responses on this one but, IMHO, any good name brand 100% synthetic premix should work fine. As for ratios, it depends on your type of riding. In a 300exc, if I were doing woods riding, I would go with a 50:1 mix. For wide open running like desert or MX, I would go with 40:1.
I'm using Spectro SX 100% Synthetic now because I like the way it smells and it burns real clean.
I use Mobil MX2T (pure synthetic). I used to use Golden Spectro.
The only way to know what burns clean is by removing the top end and inspecting the power valve. I did not like the way my power valve looked when I replaced the last top end. I won't be able to tell how clean the MX2T is until I tear it down again and inspect the power valve.
I got the tip on Mx2T here on DRN. I am pleased with performance, and can't wait to take a look at that power valve!
Another plus, I order it from www.avlube.com, so I don't get gouged by the bike shops, by buying their 2-stroke oil pick-of-the-month.
They're right, you'll get a million different responses on brands of oil to use. My belief is that any quality, name-brand synthetic is fine. I've been using Silkolene Pro-2 fully synthetic in my 300 MXC and it seems to burn cleanly and doesn't foul plugs. I've been mixing it at 40:1. The main reason I run the Silkolene is because my dealer has the same motorcycle and runs the same oil, and I wanted to copy his jetting specs and I figured the best way to do that was to run the same oil. So far so good. I haven't had the top end off yet, so I can't verify how cleanly it's burning although my dealer swears it's the purest burning stuff he's used. But again, you'll get a hundred different tips and testimonials on two-stroke oil. Pick one you're comfortable with and I'm sure you'll be fine.