Which oil for an old petrol turbo

[Ben]

I'm trying to decide which oil to get for my Volvo redblock turbo engine. There are many threads on the subject but many different opinions, some say fully synthetic 5w40, some say 20w50 mineral. 
15w-40 semi seems to be quite a common suggestion. Is there a reason you'd want a thicker oil on start up, rather than a 10w? I've got a gulf competition 10w40 in the shed but won't use it if 15w is needed