Noisy lifter 1990 Chevy K1500 4.3l v6
 

I had a lifter get noisy on Thursday on my way home from shopping. Seemed to be noisy around 55mph (5 speed manual. Was not constant came and go, if I dropped speeds it seemed to lessen or go away. I put 5 ounces of Sea Foam in it and haven"t heard the lifter again. Don"t drive the truck much as it"s my plow truck and also used to pick up lumber or bigger objects. I"m thinking of adding some atf and then run it for an hour or maybe a hundred or so miles. I was going to add a quart, but my level is now at the full mark, do I need to try and drain some oil off before I add the ATF. Any other recommendations?
Thanks,
Bill