Oil filler cap with white "gunk" on the underside is most likely just condensation mixed with oil.
This can be common depending on where you live and your driving habits.
When engines cool off air from the outside is pulled in, it can have a high humidity because it is warmed by the engine, as engine cools more the air cools and any moisture condenses on the colder areas like the plastic oil cap(similar to a cold drink on a hot day gets water beads on the outside of it, condensation).
If you drive the truck for at least 30 minutes on each trip most of the moisture will evaporate, but if you drive short trips, under 15 minutes the moisture isn't all burned off.
I would also check the Vent Tube, from valve cover/oil filler to Air Plenum, and the PCV valve and tube, these can be clogged with that same condensation "gunk", clean them out to reduce condensation.
High oil use, without obvious signs of a leak, can be valve guide seals or rings, a leak down test checks the rings.
Basically you put a cylinder at it's TDC, so both valves are closed, then apply compressed air via the spark plug hole, with a pressure gauge attached.
Then watch how fast the pressure "leaks down" past the rings.
A quicker test but not quite at accurate is to seal the PCV valve opening in the valve cover, and then the Vent Tube opening in the other valve cover, pull out the dip stick.
Start engine, check the amount of air coming out of the dipstick tube, this is pressure going past the rings from each cylinder firing, it is call Blow-by and a bit of it is normal, a few lbs of pressure.
If you have a lot of pressure then yes rings are leaking more than normal.
If you think it is valve guide seals you can replace those just by removing the valve covers.
Are you losing coolant?
If so how much?
If you are not losing coolant then that "white" smoke is oil being burned.
Head cracks or blown gaskets always cause over heating because of coolant loss.
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