I had the same problem. I used Marvel Mystery Oil, but I left it in for 3000 miles the lifter got quiet at about 1600 miles. I had changed pushrods and rockers because they had some wear, more than I thought they would have with just under 90,000. I did this before MMO and noticed a sludge film on every thing under the valve cover. I did my oil change last weekend and pulled the valve cover spotless,I mean it looked brand new! no sludge, all of it gone. I am going to keep using MMO every oil change and I changed from Rotella 10-30 to 15-40 and use Wix filter. My engine is now running like it did when I fist built it, that stuff is great!