[Aussie Mike]

This is a common thing I've seen on LS engined cars. Commodores do it too.

I suspect it's to do with the design of the hydraulic throwout. Here's my theory:

Most cars with a clutch fork and slave cylinder have the slave on the outside of the bell housing. The cylinder bore of the slave is protected from the elements with a rubber boot over the pushrod.

The LS motor has a hydraulic throwout with a large ring shaped piston about 2" in diameter. There is no dust seal protecting it from the ingress of dirt and dust. There is also a lot of surface area on the sides of the piston and piston bore compared to a conventional slave cylinder. A lot more area for dust and dirt to collect. Each time that piston moves back and forth it tracks some of that dust and dirt back into the areas behind the piston seal. This discolors the fluid.

Cheers