They use castor because of the ability of it to protect the engine during the first runs where wear is highest. You will only need around one ltr of fuel with castor to run it in, had to do the same with a Magumn 160 twin and then after run in I only use Klotz as that is the only oil that I have used since the early 80's. If there is a bike shop around there you will be able to get castor there.

Cheers