GTA Oil

I want to give my GTA an oil change. The handbook says API SE Standard oil (Multigrade) in 15w40 - 20w40- or 20w50. Is it necessary to stick to normal multigrade oil, or is it better to use a Semi-Synthetic or full Synthetic oil ?

search for "oilman" threads. He is the owner at Opie Oils.
I think he says that the "base oil" is very important and Ester is best.
Fuchs Titan Pro S, 10w-50 and it is Ester based Fully Synthetic.

Thanks Simon

I have used Mobil 1 fully synthetic for many years. I think it used to be 15w-50, now I use 10w-60.