I had an old suburban with a 350 where the lifters would start sticking and cause it to run cruddy and make a little noise. Every now and then about 100 mi. before oil change (that's when it was about a quart low too) I'd add a quart of Dextron to the engine oil. I was told years ago that it's really high detergent and it must have worked because the noise went away and the engine ran smoother. I can't swear that this is approved by anyone but it was my engine I did it to and I didn't have anything to loose.
Tom
