When I did my PTU and RDU with about 60K on the clock, we ended up double-dipping on it. The factory, erm, "fluids" were thick, chunky and pasty and a pain in the ass to extract, so my mechanic friend assisting me suggested sucking out as much as possible, refilling with fresh fluid and then driving around for a period of time to allow the new fluid to mix with the old, then repeat the process a couple of weeks later. When we performed the second service, the extraction was much easier and more thorough. Doing the process twice is a bit of a pain in the ass but I do feel it's worth it to ensure it's done thoroughly.