Spoke to Guy at Opie Oils, and got an answer to my question if anyone else was interested. The millers and Silcolene are both top end oils, but the 10w60 is arguably a bit too thick for the Zed engine, and 5w is more suited. The engine will take around 4.5 litres. This retails at just under £50, and is sold by Opie Oils for around £45 so £35 is a bargain.
If anyone else is interested, I'd take 2 at this price. Only 8 more needed for group buy