I am about to open a can of worms and can do no better than repeat a post from someone else from the old website in 2011. Of course the oil recommendations may have changed since then. My advice would be to get a definitive answer form Ian Allan Motors as to what oil they use.
"As you probably know, for Corvette C5s the GM recommended oil in the handbook is Mobil 1 5W-30 which conforms to specification GM4718M. This has not been commonly available, so with approval from GM, most folk, including me, have been using Mobil 1 0W-40.
My oil change was due and I was pleased to find on Halford's shelf, Mobil 1 ESP Formula 5W-30. At last, the right grade, off the shelf I thought. I duly bought some but before installing, as there was no mention of GM 4718M, I thought I'd check with Mobil 1.
Their response was as follows. "Unfortunately Mobil 1 ESP Formula 5W-30 doesn't meet GM 4718M specification. GM 4718M is a specification required only for GM, Chevrolet & Corvette cars made in USA and therefore we have a number of engine oils with such spec in the States but in Europe we only have Mobil 1 0W-20 and Mobil 1 5W-30 (in Russia only), due to limited presence of such cars. As far as I know Mobil 1 ESP 0W-40 is the best solutions for cars requiring GM 4718M, yet it doesn't meet it so I would use it after warranty only. Going for Mobil 1 0W-20 may be risky in your car because it's too thin even if it meets GM 4718M."
Following on from this I checked with GM UK and this is what they had to say.
"GM have developed the DEXOS brand of oil across the world. In the US, DEXOS 1 is only avail and meets all the GM Gasoline
requirements, today and retrospectively. In EU, DEXOS 1 is not sold, but DEXOS 2 is , and this DEXOS 2 oil meets the
Diesel requirements AND GM Gasoline requirements, inc. Corvette C6 and
older models.
Quote from GM Bulletin #11-00-90-001: Global Information for GM dexos 1 and
GM dexos 2 Engine Oil Specifications for Spark Ignited and Diesel Engines,
Available Licensed Brands, and Service Fill for Adding or Complete Oil
Change - (Mar 14, 2011) ,
"
dexos 2™ is approved and recommended by GM for use in Europe starting in model year 2010 vehicles, regardless of where the vehicle was manufactured.
dexos 2™ is the recommended service fill oil for European gasoline engines.
DEXOS 2 is avail from Vauxhall/Chevrolet Dealers."
And so folks, it looks as though GM are now telling us to use Dexos 2 as service fill. I returned my £50 a gallon Mobil 1 and bought some genuine GM Dexos 2 5W-30 from my local GM dealer. It seems to meet most of the grades of the Mobil 1 0W-40 but additionally it has the all important GM approval. And yet, it cost me jus £18.00 a gallon. Work that one out if you can. Have I misread the situation or has anyone anything to add? Thought you'd all like to know."