I'm not sure how many of you watch Hot Rod TV (or is it Horespower TV now?), but a week or two ago they changed the engine, tranny, and rearend oil in a fourth-gen camaro and gained 8 rear wheel HP on their chassis dyno from just that change. They used Royal Purple fluids everywhere. Honestly, I would've never believed it had I not seen the dyno.
It was a manual transmission car, IIRC - I'd guess that an automatic would benefit similarly.