Alright my EL XR8 is specified to have 15w40 oil. But now the car is different.
Instead of E7s heads and pressed rockers it has GT40 heads and roller rockers and an AU3 XR8 cam.
Do I need to change the rating of my oil to what the AU3 XR8 has?

What is the Fords rating of oil for AU3 XR8?

It doesnt change one thing, the oil is fine. Roller rockers require less oil and in the T3 motor oil to rockers was restricted slightly to maintain good oil pressure to bearings making very important to not use to thick oil but even 15/40 I would use on the T3 in summer or with a few K's up.
It'll all be good just use quality oil and change every 5 to 7000 with a filter and it will give good insurance.

Ok cheers for that.