After 80k miles they will clog up regardless.
Not sure that you can put a fixed mileage on that '6'. If an engine has had an oil change every 5,000 then surely you would expect those hydraulic tappets to last longer then one thats only had one every 10,000? Of course there are other variables that can be thrown into the question too, such as how hard the engine has been thrashed, what oil has been used etc..........
As a sidenote, the ones in Hazels Audi (VW 1800 engine) have done 178,000 miles and are fine.......I've done the valve guide oil seals but the hydraulic tappets are nice and quiet and still going strong
Just a shame she can't keep petrol in the tank.........doh