I got my ZZ4 in (I think it was) 1999, so the engine is ~20 years old. Probably ~50k miles for the amount of mileage I've been putting on it (~2500/year). Lately when the car sits for a week or more, one of the lifters starts clacking upon start up for about 20-30 seconds before finally settling down. The engine has had nothing but Mobile 1 synthetic in it from the get-go. Is there some additive you guys would recommend to resolve this?