As I am new to this forum, I don't know whether anyone else has replied to your post, but rapeseed oil is marketed in the supermarkets as Canola oil, as a fertilizer it isn't too bad, much better is well rotted cow manure, chicken manure (marketed as Dynamic Lifter here in Australia) and fish emulsion. Osmocote - along with another one - Nutricote is a good fertiliser, but remember, it is a chemical fertilizer not a natural one, and we have found here that prolonged use causes a salt build up on the pots, so could possibly cause salt build up within the soil. We have found the best way to fertilize is to use different fertilizers at different times, that way the trees get a variation of minerals. Hope this bit of information is of some use to you.

TinanaTaffy