I went ahead and (semi) blockheadedly sucked up SeaFoam through the brake vacuum booster line on My '05 GTP to do carbon build-up cleaning. I think it's OK to do, but not quite sure. First time for any cleaning for it; 121,000 mi. The engine did seem to run smoother. I'd guess changing the s/c oil would be smart, but i just did it about a month ago, so I'm likely to blow it off (using synthetic 5W30 now). Open to comments...
Update: It is definitely smoother and is noticeably more responsive from off-idle; practically like a new car. The throttle body should have already been reasonably clean. I let it sip through a whole bottle; then sat ~1 hr.
