What oil, changed how long ago, with how many miles. What's the oil
change history previously? I'd be very suspicious of contaminated oil
that's been there too long, and is too thick and gooey when cold.
I'd warm it up and change the oil. If it was my call I'd put some MMO
or rislone in for 20 minutes or so before dropping the oil, but there
are others who will definitely disagree with that.

How many miles on the engine? Aside from first checking the oil level, it
may be sticking valve lifters (gummed up from old age, not to mention lack
of clean oil -- when the engine gets hot enough they loosen up a bit). One
thing you can try is dumping a quart of ATF into the oil just before your
next oil change (like just before you drive to the garage or otherwise heat
up the oil for the change). ATF has a variety of cleaners in it that will
help get much of the crud out. (I actually put it in a week before changing
the oil on my old Dodge truck, and it was amazing how much dirt came out.
ATF has a scary low viscosity, but that's not too awful in the dead of
winter, especially since it also has some esoteric high pressure lubricants
& metal treatments.) If a good internal cleaning doesn't do the trick, check
to see if the lifter clearances on this engine are adjustable (some engines
not, and some yes -- you can look under the valve covers or check with a
manual or good mechanic). If they're adjustable, adjusting them is easy.
With the engine fully warmed up and idling, you loosen the retaining nut (at
the top of the post holding the rocker arm) with a socket wrench until you
clearly hear a definite rapping noise. Then you tighten it down, one gradual
quarter turn at a time (too much too fast & the engine will stall) until the
rapping noise has gone, then tighten down another half to a full turn.
Because valve adjustment is labor intensive and doesn't involve ripping you
off for any parts other than new valve cover gaskets, most mechanics never
do them, and if they're adjustable they *will* get too loose over time.

Could be an exhaust leak between the manifold and head. As the engine
warms up the parts expand and seal the leak. Retorque the manifold
bolts, using a torque wrench, when the engine is cold. Definitely change
the oil every 3000 miles, 2000 for frequent short trips.

Gyz