Use mobil 1 motorsport 15/50w you cant go wrong its 100% fully synthetic it costs around 35 quid. I use it in my motor and will not use anything else. I want to look after my engine as its only done 4500miles on rebuild. Also if you speak to Burton Power they will tell you the same. If you have spent alot of time and money on your engine then use the expensive fully synthetic oils so your engine will last