lifters suck.. or they are supposed to suck anyway.. they are 'sposed to 'pump up' by sucking oil in to themselves and charging the two chambers inside them... one chamber (the lower one just above the roller) is spring loaded.. that spring is only strong enough to push the lifter piston up when the engine is not running.. Once the engine starts laying some RPM's into it via the cam lobe, the lifter doesn't stand a chance to keep up without the hydrolic action of the oil being pushed out and pulled in.. so you get a 'tap-tap-tap' which is annoying.. and somewhat dangerous to an engine..
it's dangerous because the travel of the lifter rods is reduced the same amount of travel the lifter piston is being limited.. which means your valves aren't opening the distance they are 'sposed to.. which means your engine isn't breathing like it's 'sposed to.. the effected cylinder stops 'pulling it's own weight', and lugs on the engine, which tosses balance out the window.. It's not like it's going to kill the engine in short order, but you're losing performance and economy..
Oil is 'sposed to fill those lifters pretty quick. Oil is 'sposed to drain pretty slow out of them after the engine stops.
the issues a lot of fella's have are impurities in the oil floating around and being sucked into the lifters- or, sucked up to the lifter port and blocking (or slowing) their ingestion of oil.. or those same little chunks getting caught up in the check ***** inside the lifter and either leaving the port open or clogging it.. you get that nice little tic-tic-tic that increases tempo with RPM's until the lifter either fills or until you stop the engine..
Before you tear into the internal engine swapping out lifters (if that is in fact the problem), you can swap oils and filters.. swap to a much lighter conventional oil for a little bit (the detergent will theoretically suspend the chunks), and swap oil every 500-700 miles, for at least two rotations.. then put the rec'd weight back in it.. You're hoping to clear the clingy little chunks of crap out, and see if your lifter will recover..
by the way- someone suggested an exhaust leak.. exhaust leaks tsk-tsk-tsk more than tic-tic-tic.. but still can be hard to determine.. The absolute best way to find an exhaust leak in my opinion involves an entire can of seafoam in the vacuum line leading directly to the intake.. OOOoooh- all the pretty smoke--should come out of the end of your exhaust tips exclusively.. if it comes from somewhere else, you've located your leak..
swapping oil a couple of times can absolutely recover a lifter..