20w50
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I know the newer cars and trucks have much smaller holes in the heads than the old ones so they recommend the oils like 5w-30 but an old truck like yours should probably have at least a 30 weight oil and It won't hurt it to put in 40 or 50 weight oil. If the motor has a lot of miles on it, the high mileage oil will help keep it from wearing out sooner (they put a little bit of brake fluid in it to make the seals expand). The most important thing is to change it on a regular basis. Good Luck!
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That old truck probably wont be too picky on which oil it needs.
Depending on the conditions you are operating it and if the engine has alot of internal wear,
- I would probably just use a 10w30 in fair weather climates.
-It is it really hot in your climate maybe go a bit thicker like a 20w40
- if you are in sub zero cold 5w30 would be better.

Overall, The 10w30 is probably your best bet.

Thats a cool old truck! Take good care of it and she will serve you well!
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10-w40 Quaker State for older model cars and trucks. Durin the winter time you need to run a heavier motor oil for the engine parts to remain lubricated and so they don't freeze up. Durin summer run a 10-w30 to help keep the motor runnin cooler in higher temps so the truck doesn't get hot. Check your transmission filter and your radiator make sure you don't have any little scraps in the pan. Run your truck hot to check transmission @ change your radiator coolant once a year. If its the original radiator it may stay original but like anything with time you might want to think about modifying to a little bit bigger radiator.
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Mechanic

visit http://www.mygmlink.com
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retired Chevy dealer assistant service manager

In my old vehicles, I use "20-50 Valvoline". The oil pressure seems to stay higher and more constant with that oil.
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The industry standard is still 10w-30,but this is because motor oils have changed while perceptions have not,most motor oils are blended with the same levels of petrochemicals as the next to insure the ability to meet the minimum standards of today's automakers,having said that you have to consider the offerings of synthetic and synthetic blend motor oils,due to the age of your truck you probably would not want to use most of the synthetic motor oils however Pennzoil,Quaker State,Castrol,as well as others do offer high mileage motor oils in synthetics and blends,I would recommend a high quality 10w-30 conventional motor oil,in summer use witch is recommend for temps of 35f to 100f in most gasoline engines, or 5w-30 for temps of 10f to 70f although a lot of newer cars require 5w-30 and 5w-20 all seasons due to close tolerances.

Petroleum Distributor for 22 years.
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