You can use the GTX 10w-40, but in terms of an engine like this that can run hot is really not up to the job, especially if the car is modded or driven hard.
The GTX 10w-40 will have trouble staying in grade after or around the 105dec mark, dont be fooled that a lower quality mineral based oil is as good as a genuine synthetic if changed more often, this is just not the case.
The good thing about the 10w-50 grade, especially the Silkolene one is its such a good all round grade, covers everyhtng from, mods to hard use/track (happy running at 150dec and above) and also ideal for day to day driving.
Cheers
Guy.