Does anyone know if it is OK to switch from Castrol synthetic to Mobil 1 in an M4? I'd never thought about changing oils, but now BMW is now partnering with Shell instead of Castrol. I assume the M3/4 (or any BMW up to this point) has been engineered and developed using Castrol synthetic oil/lubricants, but does it matter? I don't know that either product is actually 'better' than the other, I've always used Mobil 1 based on manufacturer recs (Porsche). Do only the SAE specifications matter in regards to choice of oils? Thanks for any input and Happy New Year!