Its not that the GTX is too thin, more like it isnt up to the job.
With mods like this your really looking to use a quality synthetic, 5w-40 is fine unless engine/oil temps are extremely high.
Brand wise, I would go for either Silkolene and Motul as these are ideal for track/road use.
http://www.opieoils.co.uk
Cheers
Guy.