This is a TSB for the 2004s. Saw your post then stumbled across this by accident:
∑ Informational - Discolored, Dirty, Dark, Sludge and/or Grease in the Clutch Fluid Reservoir #04-07-31-002 - (Feb 16, 2004)
Discolored, Dirty, Dark, Sludge and/or Grease in the Clutch Fluid Reservoir
2004 Pontiac GTO
with Transmission, Manual 6 Speed, Tremec, 85 mm, 2.97 First, 0.57 Sixth, Overdrive (RPO M12)
It may be observed on some vehicles that the clutch fluid in the reservoir appears to be discolored, dirty, have sludge or grease contaminates in it after being in service for a period of time.
The level of discoloration will vary with factors such as driving conditions, under hood temperatures etc.
This is a phenomenon that may occur with time due to the interaction between the clutch system rubber parts with hydraulic clutch fluid.
The small amount of sludge or grease that may be noticed is, in fact, grease that has migrated from the master cylinder assembly. The grease is used during assembly of the master cylinder.
Engineering investigations of returned fluid samples show that discoloration and grease in the fluid DO NOT negatively affect the performance of the clutch hydraulic fluid.
Field reports indicate that where clutch fluid and or clutch system components have been replaced to address this condition, the vehicles have returned with the same condition causing a repeat repair and a dissatisfied customer.
Discolored clutch fluid should not be replaced in a normal operating system.
For Clutch System Concerns and Diagnosis, refer to Diagnostic Starting Point - Clutch SI Document ID #1338362.