Now that you have specified an 8 cylinder engine, I still
suspect bad/sticking lash adjusters or low oil pressure to
the heads.  The oil passages are relatively small and may
become restricted over time if the oil is not changed
frequently enough to prevent sludge buildup.  Another
primary cause of sludge buildup is operating at cool
temperatures as in many short trips or a bad thermostat not
bringing it up to normal operating temperature.  If sludge
is the problem, you may be able to help by using an engine
flush additive to clean it a bit.  If you are using anything
other than a high quality 5w20 oil as recommended, the oil
may be a bit too thick to circulate with restricted
passages.  With extented hot operation, it "may" get better.
If replacing the lash adjusters and going to a lighter grade
oil does not do the trick, you may be in for oil pump
replacement or engine overhaul.  If it is the same follower
coming off, I would start by replacing the lash adjusters on
those three, running an engine flush, and putting in a good
quality 5w20 oil.  If that doesn't do it, you are in for
heavier repairs.

Lugnut