If it's for a series 3, try and get GL4 spec not GL5 as (I believe) that GL5 can leach out the brass from the syncro mesh gearing. To be fair it's not very easy to get hold off!! I've bought EP80/90 GL4 (comma oil) in the past from Wilkinson's which was a fraction of the price that Halfrauds wanted for it. Agricultural supplies might also be able to help.