Sitting comfortably.... The recommended oil is an SAE 10W/40 multigrade of API spec. either SF/SG or SH/SJ. These will probably only be found in the motorcycle section in places like Halfords as most car oils although showing 10W40 will be of different API specs. A semi-synthetic is fine and there's no benefit in spending extra on a full-synthetic. The general view is to buy a recognised brand such as Castrol, Silkolene, Shell, Mobil, Motul, Morris etc. Any good brand of the right spec. will do just fine. You'll pay premium prices at bike dealers and should be able to get a better deal off the internet (Ebay?). Some companies will do you a complete service pack, as you've already seen at very good prices.