I'd run synthetic. Castor-based oils will tend to gum up powervalves. I've been using Torco GP-7 full synthetic, and it works fine. Motul 800 seems pretty popular as well. Any good synthetic should be alright though.
i agree. from all i've heard, castor offers slightly more maximum rev protection for highly stressed mx bikes, but clogs up powervalves. you'll get enough hours between top-ends on a 300 that you'll appreciate the pv cleanliness. i've used mobil1, belray h1r, and silkolene comp2t. all seem to work great. my bike has around 600 hrs on it, and i'm getting ready to do my first top-end, just because i'm getting nervous. the bike still runs better than new.
I've used Golden Spectro (semi-syn.) protects real well, runs dirty, spooges alot, approx. $4.75 for 12oz. Champion WP2 (semi-syn) cleanest burning semi-syn. I've seen, protects real well, spooges less than Spectro, approx. $4.25 for 16 oz. Red Line (syn.) very clean burning, good protection, minimal spooge, $8 for 16 oz. ! Mobil 1 MX2T (syn.) burns very clean, good protection, the least spooge I've ever seen, $3.49 for 16 oz. available at Auto Zone ! Amsoil Series 2000 (syn.) burns very clean, good protection, minimal spooge, approx. $8.25 for 32 oz.
All are cheaper by the gallon !
I was using Mobil-1 MX2T until I read several good reports on the AMSOil 2000 full synthetic 2 stroke oil and I found an internet dealership that will sell it at cost and even with shipping it runs only $5.75 a quart instead of the $3.50 a pint I pay for MX2T.
The link to the dealer that will sell it at cost is: