i know this topic has been done before, but i just bought some oil (castrol R4 superbike) and it has a 5w40 weight, where the bike manual recommends 10w40. its supposed to be the best synthetic motorcycle oil castrol has, but it only comes in 5w40. im just wondering if i should return it or if it will still be ok. it says its good for all 4 stroke motorcycles. i know the 5 is for a lower temp. and it is a thinner oil, but why would castrol make only that one kind?