LG, when I'm stuck with store bought fuel, I just top off the jug with castor. There's always some airspace in the jug to do that. I've never encountered 8% oil fuel, then you would want to transfer some of that fuel to an empty jug and then add enough castor to double the oil content. A little bit too much oil doesn't do any harm except to your wallet, it's the most expensive component. Most LHS fuel around here is Cool Power.
I used to buy racing fuel ingredients where that were listed in the yellow pages. It was a VP distributor. They sold 5 gallons on up of nitro and methanol and gallons of oil. Now I just go to the Chevron, Standard Oil dealer down the road next to the Co-op and they have the stuff. I bought 55 gallons of methanol for $160 just the other day. I had to mail order a couple gallons of castor from Sig, they included rumpled up drawings of the Hog Bipe, too! IIRC, the oil was about $50 delivered. I'm still working off an old 5 gallon drum of nitro so I'm not sure what it's going for now. My bigger engines don't need nitro, but even 5% makes a big difference.
No place where I bought ingredients checks who I am, they just want the money.