Change the oil and substitute one quart of oil for diesel fuel. Dont run it
long, cause you know diesel is combustible.. LOL But you can run the engine
for about 10min and it wont hurt it. DONT rev it up either, that can harm it
due to the low viscosity of the oil/diesel mix.

If it aint clean enough after the first time, "rinse and repeat".. LMAO...
All kidding aside it should get the job done for you. Just done use regular
gasoline.

On a side note. If you can see tons of build up in the valve cover or where
ever you see it. Imagine what your oil gallery looks like, those drilled
passages in the crank, and the oil passages in your push rods... HMMM, makes
ya wonder how clean the engine really is huh? Makes ya wanna tear it down
and clean it up right don't it....  Just some food for thought...

Ford Tech

ATF makes a good cleaner and you can leave that in there and drive it. (one
quart, not six)

You do not want to use anything that cleans it too quickly.
That may cause problems by plugging oil passages, the pickup
screen and filter.  Put it on short oil change intervals
like 1K miles for a while using 1 qt of ATF with the oil.
Also, make sure the engine is running hot enough to activate
the "detergent" action of the oil.  This generally requires
the temperature of the oil to be above 140 deg/f. Keep in
mind that you will probably never get all of the gunk out
short of rebuilding the engine so don't be too concerned
about it if all is well otherwise.  If you get it too clean
at this point, you may be rushing overhaul time.  Every mile
you drive before overhaul is cheaper.

Lugnut

I had a '69 Bronco, added a quart of kerosene (basically the same as that
Gunk additive), ran it for 10 minutes and you wouldn't believe the crap
that drained out. If you can get it, biodiesel will really clean that
sucker out being a better solvent then petro based fuels.

I suspect you are looking through the breather on the valve cover and
seeing heavy deposits of gunk...right?  Signs of bad maintenance and
short trips.  This is one of those;  "if it ain't broke, don't fix it"
deals.  You can add (whatever) to the oil and it may help, but I
wouldn't.  Use a good detergent oil and a good filter, I use only Wix,
and change with religious regularity.  Check/change the PVC valve.
Clean the breather and make sure the hose between them is good.  I've
got the same truck/engine.  The valve train on mine is pretty darn clean
for the 26yrs it's been my daily-driver.  The 300cid is near bullet
proof and maintenance is everything.

Dave S(Texas)

Thanks for all the good suggestions.  I will let you know that come out.
HAHAHA