[Vwdiesel] Oil flow problem??

[Craig Osborn]

About two weeks ago, I started my 81 pickup in 18F temperatures.  It started
but ran rough which is usual behavior in such temperatures.  After warming
it up for about five minutes, I proceeded to set out onto the road.  Almost
immediately the engine started to squeal.  Initially, I thought it was a
loose belt and ignored the noise.  After a 1/2 mile or so, the oil light
came on and the engine lost some power.  I realized that the squeal was
likely not a belt.  I happened to be next to an auto parts store, so I
pulled into the lot and all of the sudden the power picked up and the oil
light went out.  I pulled into a parking space and checked the oil level.
It was about 1/2 quart low.  I added a 1/2 quart of oil and restarted the
truck.  It ran fine.  It did not seem to be damaged from the incident as I
was able to drive it the rest of the way home (54 miles in a mountainous
region).
I drove the truck regularly over the next two weeks without incident in
temperatures ranging from 25F to 40F.  The engine ran smoothly with normal
power, almost no smoke (fuel or oil).
Today, I started the truck in 10F temperatures and let it warm up for over
20 minutes while I cleared snow from my driveway.  The truck idled smoothly
the entire time and came up to full normal operating temperature in the 20
minute period.   I decided to take the truck out and fill the tank in
preparation for my Monday commute.  I  made it about three city blocks away
from my home when the oil light came on and the engine started to squeal.  I
lost braking power as if the vacuum assist system stopped working ,and the
engine started to run rough.  This time I shut it down quickly and parked it
on the side of the road.  I waited 30 seconds and tried to restart it.  It
cranked very slow like it was overheated and close to being seized and would
not start.  I walked home, ate dinner and walked back to the car about 45
minutes later.  It started normally and ran fine the whole way home.  The
oil level is at the full mark.
Is it possible that the oil is congealing at cold temperatures and not
allowing the oil to circulate?  It has been about 9000 miles since the last
oil change.  
Additional info--
- I use a blend 10w40 diesel oil in the truck-
- The truck is driven about 500 miles every week and uses around 1/2 quart
of oil each week.    
- The side of the engine block is wet with oil from the head gasket down.
It has been this way for the past two years.  I regularly check and fill the
oil.
-- The antifreeze does get discolored after around 30,000 miles.  (Head
gasket??)
- There is sometimes, but not always a small amount of pressure in the
coolant reservoir (Head Gasket??).
- I do not see any coolant in the oil. 
- There is no unusual water vapor in the exhaust at startup (other then
normal vapor when it is cold outside). 
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Can anyone help diagnose the problem?  I am afraid to drive it to work.
 
Craig