Could someone advise me which oil is best to use in my 20v marea as it will be left for several months at a time between uses and then used for a couple of weeks (left in france at renovation site).Any other advice on best way to leave the car ie tyres battery etc would be great. Thanks

A good synthetic 10W-40, Fiat recommends Selenia 20k but it can be a bit hard to get hold of.

Pump the tyres up to well above their nominal pressure to avoid them flat spotting. (use a foot pump). Leave it somewhere out of the rain and well ventilated.If you can get one an intelligent trickle charger is a big help. Conect it then just leave it on. I have an Oxford (Oximiser) one on my MGB and that car starts on the button after standing for months. Grease the battery terminals to stop them corroding, petroleum jelly is usualy advised but Lithium grease seems to work fine. Using synthetic oil is good, especially since Fiat recomend it anyway, just make sure it is at least the specified API grade. Oh. and let the tyres back down to the right pressure before you drive it!
Oh and the other problem is the clutch, it will tend to freeze to the flywheel, really nothing you can do about that, with the MG I operate mine about once a week through the winter.

Or jack it up off the road wheels if you can...

One other thing. Drain the fuel tank. If you are going to leave it more than a month the fuel will evaporate and leave you with thickened fuel.

Interesting comment about the shelf life of petrol. I keep as little as possible in the MG tank if it is going to sit for while, however even after standing all winter the car will always start and go fine in March when it goes for it's MoT. The fuel pump runs for a while as the carb float chambers are as empty as a bank account the Chancellor of the Exchequer has found. Yes it has lost some of its more volatile fractions but you might not even know that on a modern EFI engine. I never bother to put oil in the cylinders and when the head came off last year there was no pitting on the cylinder walls and some honing marks were still visible. I always used to laugh at that Magnatec advert showing an engine grinding itself to bits on a cold start. I once picked up a rocker shaft from under the bench where I had put it down 10 years before, sliding a rocker along revealed the oil (not even semi synthetic) still filling the bearing space just waiting to do it's job. Hope this cheers you all up a bit.