This engine is well known for having timing chain problems, especially if the oil hasn't been kept clean. As it came from a known source, what's its service history like?

I certainly hope rcbikeracer's not right about needing doing every 40K, mine's coming up to that now, although it's had annual oil changes (5K or less).

It's quite easy to check by removing the cam cover and measuring the tensioner protuberance. More than 1cm IIRC and it needs a new chain. There's plenty of information on the interweb thingmy about it.

What is the best oil to use on a engine with a timing chain?

Clean oil.

Before going to the expense of doing the timing chain, try replacing the O2 sensors on the exhaust. There's a chance they're stuffed if the exhaust was off for replacement.

Ok thanks, we will try cleaning/checking o2 sensor contacts (and the clean oil ha!) i did wonder about that with it being on/close to the exhaust but we've just spent £198 on an air flow sensor and didnt want to be spending any more on something that may or may not work, I must admit tho it does feel better in performance since the air flow sensor was replaced apart from the hesitation on cold start and the light isnt on constantly just ocassionally.

We definatly wont be spending all that money on replacing a timing chain no way...Nissan make my blood boil, Why wasn't it a recall if its a notorious faultly chain, They quoted £85 to get the codes read.. I think not when i could get it done for £40 elsewhere grrr! Rant over :-)