v8snerlo

OK guys, I 've got a question for you. I have an AUII XR8 (200kw). The manual says to use 15/40 oil. I have been using Shell Helix Ultra synthetic for a little while now which is 5/40 rated. Some of the guys up here in the XR club of Qld have started using Castrol formula R (0/40 or 5/30 I think). They say that the castrol oil really quietens down their engine for some sort of reason. I was thinking of giving it a go because the old windsors aren't exactly the quietest in terms of engine rattles and ticks. So, getting to the question, and not knowing too much about oils, is putting Castrol in the car going to make ANY kind of difference or should I just stick to Shell Ultra. I thought that synthetics are synthetics and that they would be pretty much all the same. I know the question is a little confusing but noone ever talks about the old windsors anymore. Interesting to here all the oil problems that the Bosses are having though. My 5.0 litre doesn't use a drop (touch wood). Any advice would be greatly appreciated. Thanks guys.
Leon.