valve tapping

i had a 01 explorer with 94420 miles with 4.0L SOHC. when i bought it it had 89000 miles. i had got some advise from a mechanic to use synthetic in it right away to prevent bulid up of deposits. after i did that the vavles started to tap when i start it up in the morning for a couple of seconds and it stops. if i rev the engine, there is no tapping noise. but when drive it and i pass 2500 it starts tapping again. so i tried a thicker sythetic and still is tapping. ohh i started with the recommended 5w30 and tried a thicker 10w40 sythetic with one qt. of lucas. can i switch back to regular motor oil or will that ruin something? now it sound like a sticky lifter and have tried the qt. of tranny fluid and 4 qts. of synthetic oil for 1200 miles and still i have tapping valves.
will i have to replace my lifters or is there an easier fix?

The way to end valve tapping:
http://www.auto-tune-up-and-repair-options.com/valve-lifter-noise.html