For the last year I've been driving around with the dreaded STI rear strut clunk, and I finally got off my lazy duff and fixed it.
I removed the two rear struts, scooped all the old yellow grease out, and replaced it with Mobil 1 synthetic grease. Each strut got about a 3 cm ball of grease rubbed all up and down. I then put everything back together and voila, no more clunk. It is practically like riding in a Cadillac...well not really but compared to the previous bang bang clunk clunk it is SOOOO much better.
The old yellow grease was almost waxy. The new red Mobil 1 grease is still slick but does have a slightly stickier consistency. And I used quite a bit more than I originally removed, per the NASIOC instructions.
Also, I could be fooling myself but the ride at low speed actually improved. There is less bobble head feel. There is definitely a lack of the low speed sticktion that accompanied the clunking. It is hard to describe, but at 0-15 miles an hour it was a weird jerking feel as the strut finally moved. That feeling is gone. If I have reason to take apart the front suspension, I'll probably replace the grease in the front struts too.
Interestingly, my struts were clunk free for the first 3 years of service which is pretty unusual. My theory is that the several large speed humps I drove over every day at my old house kept my strut seals properly lubed. These speed humps were of the 10 foot wide gradual hump variety (as opposed to the 1-2 foot wide kind). Within weeks of moving about a year ago, CLUNK, I had the strut clunk. Its a theory and I'm sticking to it
Why oh why did I wait so long? It is so much nicer to ride in the car now. Hopefully this solution proves to be long term, or else I'm going to be forced to switch over to the way too expensive OHLINS, 'cause I just can't go back to the clunk.
Bottom line, if you are putting in STI struts replace the grease first. If you have STI struts on the car and have the clunk, take the time to re-grease them, you won't be sorry.