I've just bought a 90's spec (G plate) XR3i.

The guy who I bought it off claims he did a full service himself but had no receipts to warrant this, the oil on the dipstick looks clean so I'm thinking he's topped it up at the least.

Would ideally like to change the oil and filter now incase he's used some cheap nasty stuff.

What grade of engine oil should I use and are all oils similar in terms of brand?

I read somewhere that it should be 5/40, I was going to buy castrol.

Thanks.