You can mix mineral with semi-synth.
You can mix mineral with synth.
You can mix semi-synth with synth.
And any combinations of the above.
You can mix 10/40 with 15/40 or 10/30 or 15/20 or near enough any combination you like. I say near enough only becuase, unless you move to the frozen wastes or burning deserts, you are unlikely to find any of the wierder numbers on the shelf of Halfords or your local garage.
For example: If you mixed a say a 50/50 mixture of 10/40 with 15/40 you will end up with 12.5/40. If you mixed a 50/50 mixure of 10/40 with 15/20 you will end up with 12.5/30. All quite acceptable. The numbers simply refer to the oil's viscosity at 'cold' and 'hot' operating temperatures. In other words, at first start (cold) and when the engine has been running normaly (hot).
The Owners Manual shows all the oil / operating temperature ranges.
You don't have to use only Castrol oil, it's only a brand name after all. If you really want Castrol drop them an email, the address is on the bottle or on their website. They will tell you where to get it.
'Any oil is better than no oil' still holds good
