source for straight 50W oil?

Does anyone know of a source for 50W engine oil for our bikes? Since I can't find it anywhere, I use Castrol 20W50, but I have read that the straight 50W is a better choice for the Commando.

Norvil recommends the straight 50W, so I figure that it is probably a good thing to do for the bike.

I'd appreciate any sources you guys may know of. Thanks.

Mike

Valvoline VR1 Racing SAE 50 Part# 235 Exceeds API Services, SJ/CD/SH Has ZDDP additive, provides anti wear protection. Approx. $2.25 Qt. I have just started using this this summer, so far no problems. Travelerjerry.

oil reference

Thanks for the reference. I will follow up with a local store that specializes in racing parts and supplies.