I really wouldn't bother with synthetic oil. Just get the weight that
was specified for it. Your best way tomake sure the engine will last
for a while is to make sure you fully warm up the engine when you run
it (20 miles at least) AND to take it easy before the engine is fully
warmed up. Do NOT ever start the engine in the garage to 'warm it up',
it will not warm up unless you run it for a LONG time.

Organization: A World Full of Junk
Followup-To:

So long as the car is driven long enough that combustion products are
getting burned off and not thining the oil. Other's opinions will vary
some but I've never had a problem with such time periods but I always make
sure to drive it long enough that there isn't any contamination build up.

I run regular dyno mobil 10W30 in '73 maverick 250. I've used mobil 1
10W30 before on a couple of occasions. The mobil 1 was better in the very
cold weather (which I no longer drive the car in) and the engine consumed
more mobil 1 than it did on regular oil. Neither was bad consumption.. in
the range of normal, just that it consumed more mobil 1. The first time I
used mobil one it seemed to clean out what little sludge there was in the
engine, so that's another benefit provided all the seals are good. I had
no problems with seals or leaks of any sort. I switched back because I
drive the car less, the regular oil is a lot less money, and it burns less
of the regular oil.

In my mustang(4.6) I've run mobil 1 5W30 since the first oil change from new.

Naw...you're about marginal as to use, I would use regular valvoline
10w30.

The real rule of thumb for dino oils should be 3,000 miles or 4 months,
whichever comes first.  
Remember water condenses inside the engine due to temperature/humidity
change... doesnt even have to be run.

Shame you dont use it enough that you would consider selling it.  
Motorcraft are good filters

I definitely wouldn't use 5W30.  That's on the light side for that
motor.  I'd stick with the 10W40 Mobil1.  It's a bit more expensive than
conventional oil, but it has better detergent properties, won't sludge
up your engine, and will start easier in the cold weather.  And you can
probably switch to a once a year change regimen with that type of
mileage and synthetic oil.  Motorcraft filters are fine as are
Purolator.  As someone else mentioned, you'll want to avoid FRAM.

That engine is VERY easy to maintain so I wouldn't worry much about it.
 Those old C4 transmissions leak like a mofo, even under the best of
circumstances.  It probably just needs a new rear seal which is cheap
and easy to change (for someone with a lift).  You could also change the
transmission filter and pan gasket (which is probably also leaking and
very cheap to buy) while you're down there and refill it with some Type
F transmission fluid (not generic ATF).

If you have any other questions, fire away.  Most folks on the group are
pretty helpful.

Cheers,

I believe Pennzoil and Quaker State used to be parrafin based. Don't know if
they still are. Regardless, it's Valvoline or Castrol for me.

Dave

PA crude is high in paraffin.  

http://en.wikipedia.org/wiki/Pennsylvania_Grade_Crude_Oil

http://tigger.uic.edu/~mansoori/Wax.and.Waxy.Crude_html

But that no longer is the reason... at least wasnt for my empirical tests
of QS

   Actually more. They are paraffin based oil as opposed to Exxon and
Texaco etc. which are asphalt based. This is why around the 70's people
would have engines that were kludged up with the varnish like leftovers
of the wax in Quaker State.