What's the best oil to use?

[PartyJas]

I was recommended to use 5w40 fully synthetic for my reg, I topped it up after I'd bought it due to the previous owner leaving none in it. Checked today and it's used the lot but don't appear to have any leaks as there's nothing on my drive?

Also, is there any easy way to get the oil cap off as it seems to be a struggle.

How much did you put in and over what time period did it get used.?