I am somewhat new to this forum and wanted to pass along my experience with this problem. I had been slowly losing coolant of the last few months ( check and adding coolant when needed every week). Somewhat related, I was also having issues with no heat when I let of the gas.
Every time I would remove the radiator cap, the coolant was always foamy and there was a buildup of sludge at the cap opening. I never noticed a problem with the oil. I finally broke down and changed these a couple weeks ago. I had done research online ( including this forum). 1AAuto has a good set of videos on YouTube that helped me during the job.
This job is really not that hard. If you have mechanical inclination, you can do this over a weekend. It took me about 8 - 10 hours spread over two days, but I was taking my time. There are alot of little plastic connectors that like to break if you are not careful. I recommend taking pictures during the removal process ( I didn't). I also changed the plugs and wires along with a tensioner pulley. It's a good time to do so since I have 95,000 miles on it.
Needless to say, I no longer have foamy coolant and sludge in the radiator. The car seems to run better. I haven't had to add any coolant in two weeks and I am looking forward to seeing how much my gas mileage improved.
I just wanted to provide encouragement to anyone who is thinking of trying to do this job. I'd be glad to answer any questions and give my two cents