Morris Lubricants spec sheet suggests the first stop for that gearbox should be the Dealership. If you think about it, if they can't (or won't) blend an oil for it, then there is probably a very good reason why.
If you have access to MicroCat, check for a part number for a bulk buy, i.e. 5lts or 25ltrs. You may be lucky and get it at a reasonable price.