pull,
Is your engine sludged up, or are you just thinking prevention? I looked into my 42,000 mile 5.3's oil fill tube today, and it looks as clean as new. I have used Mobil 1 oil since the second oil change.
I've been doing some reading about additives on Bob is the Oil Guy.com. There's a lot of info about a product called Auto-Rx. It's pricey and mail-order only, but most of the people who post say it works. I am trying to decide whether to get some for a Saturn that drinks oil. You can go to this page and do a search without signing up:
http://theoildrop.server101.com/ubb/...?ubb=forum;f=5