Rapid oil consumption

Manufacturer specs specify one quart for every 3,000 miles. How much is yours using? Engine braking can make that amount go up as well due to the fact that your engine is producing it's highest amount of vacuum during engine braking.

How many miles on your vehicle ? Do you see any leak spots on the ground from the truck ? Blue exhaust , oil burning , valve guide seals , piston rings , PCV clogged .

Oil Consumption Diagnosis
Checks
Causes
Excessive oil consumption, not due to leaks, is the use of 1 L (1 qt) or more of engine oil within 3 200 kilometers (2,000 miles).
Preliminary
The causes of excessive oil consumption may include the following conditions:
• External oil leaks
Refer to Oil Leak Diagnosis .
• Incorrect oil level or improper reading of the oil level indicator
With the vehicle on a level surface, run the engine for a few minutes, allow adequate drain down time, 2-3 minutes, and measure for the correct engine oil level.
• Improper oil viscosity
Refer to the vehicle owners manual and use the recommended SAE grade and viscosity for the prevailing temperatures.
• Continuous high speed driving and/or severe usage
• Crankcase ventilation system restrictions or malfunctioning components
Refer to Crankcase Ventilation System Inspection/Diagnosis .
• Worn valve guides and/or valve stems
• Worn or improperly installed valve stem oil seals
Refer to Spark Plug Inspection .
• Piston rings broken, worn, or not seated properly
Allow adequate time for the rings to seat.
Replace worn piston rings, as necessary.
Refer to Cylinder Leakage Test .
• Piston and rings improperly installed or not fitted to the cylinder bore
Refer to Lower Engine Noise, Regardless of Engine Speed

It is better to remove the cylinder head and renew the valves stem seals as the vehicle has an age where the stem seals become hard where they can not hold the oil passing through to get into the valves stem and then to the combustion chamber area and get burn. Also by removing the cylinder head surely you will check for the piston rings against the bore if are worn then you should renew them,because if you fix only the cylinder head and do not do the piston rings if are worn surely you make the compression strong and weak piston rings you will have again may be worse oil consumption. Be very careful for the outcome.

Do a compression test to find the reason. Worn rings, worn valve guides, or bad valve stem seals are most likely cause.
A compression test tells you a lot about the internal condition. Cylinders should be closely balanced. Low compression in one or any cylinders should be tested further to indicate where power is lost. Then you do a wet compression test: put about a tablespoon of oil in the cylinder and check compression again. The oil will temporarily seal the rings from losing compression. If the low compression improves noticeably, it tells you the piston rings are worn too bad. If compression doesn't improve, it is a valve problem-worn valve guides, or valve stem seals.
One remedy you could try, since it runs well and doesn't smoke, is to change brands to a better quality oil, and a higher viscosity, like if you are using 5W30, move up to 10W40, or 15W40. The oil sold as for higher mileage engines will have additives to try and help with oil consumption. It often helps, not always.
You probably won't see the tell-tale bluish smoke from the exhaust indicating oil burning, until oil consumption is very high-like about 1 quart every 300 -500 miles. A quart every 1,000 miles is a concern, but many manufacturers will call that "normal", especially if still under warranty, pretty shady, but that's the facts.

Blue smoke indicates burning oil in the cylinders. Could be worn piston rings, worn valve guides, or bad valve stem seals. An engine compression test may help find it, but a cylinder leak-down test is a more precise way to find where compression is being lost. If compression is good, you just may need to replace the old rubber valve stem seals. A shop could do it without having to remove the cylinder head, using an air compressor hooked up into each cylinder while the valve train is being worked on. The air is used to keep the valves from falling into the cylinder while the seals are replaced. About a 4 hour labor cost, so expensive, but a lot less than a cylinder head removal.

Could be worn piston rings letting oil into the top of cylinder from below, or worn valve stem seals or valve guides letting oil in from above.. A shop could do a compression test or a leak-down test and give you a good idea of the engine's health. Low compression on any of the cylinders means the rings or valves are not sealing well, and could let oil in to burn. Either of the tests could point to the problem, but a leak-down test is considered the more accurate test
How much is burning? If you are burning more than a quart every thousand miles, it is a problem that needs attention. Most car makers say a quart every 3 thousand miles is not a problem and may be considered normal, although owners may dispute that as they often do. Some engines just seem to lose no oil between oil changes, but most do use or lose a little oil between changes, it is normal. If your oil consumption is on the high end, have the test done. If your rings are good, a cylinder head job (for the valve work) is much less expensive than an engine rebuild.

Excessive oil consumption would be 500 miles per liter. If that is your consumption rate you need engine repairs.

Usual cause of oil use for a small displacement engine like yours would be valve stem oil seals.

These can be installed without removing the engine. They fit over the valve stem that sticks out into the area inside the head that is being saturated by oil lubrication and oil mist. Obviously these seals need to be in good condition to stop oil getting past the seals and down where it is getting consumed.

If you need a repair shop locator try here :http://www.iatn.net/shopfinder/

( Use the world wide search in the link above. BLUE text to find one, if you are not in the USA.)

You are likely correct in the valve guide thing...A little known fact about chevy smallblock engines is that when first introduced as the 265, the cam profile was different than the later versions (283 and up) Because of the different profiles in the later engines, the valve stem gets shoved against the guide as it goes up and down, causing it to wear. If the problem was a bit worse, likely they would have changed the position of the rockers but it wasn't so they didn't.
The stem seals are a simple "o" ring. Replacing them if the guides are worn is useless. (if they were a better design though likely the guides would wear faster so they are actually good in that respect)
What you need to do is to remove the heads and have the guides replaced. BUT, I'd first remove the valve covers and make sure the oil returns at the ends of the oil galleys in the heads aren't blocked with gunk. If they are, oil will pool up inside and run down the guides. In most cases, if they are blocked, simply clearing them will get your oil consumption back to within the "normal" range.

You are most likely burning it. There is much less smoke produced these days, with the cat. converter.

A wear area on yoru vehicle is valve stem seals, when they go, oil is actually pumped out...

Also, consider a high mileage blend oil, like casteroil. I am not into mechaninc in a can, but this stuff lowers consumption and is semi synthetic...

You can reaseal the valve stems, but I wonder about guide looseness, and fairly rapid seal failure again...Still, you can get it going and sell the vehicle....

Every manufacturer has a guideline.  But a quart every 1,000 miles seems higher then necessary.  Try the easy steps first. 1.  Check the PCV valve if clogged will caused pressure in crankcase and oil burning. 2. Remove spark plugs and perform visual inspection.  IF electrode is covered in gook(technical terms, sorry), then oil is leaking into that cylinder.  Most likely cause would be faulty valve stem seals. 3. Worse case scenario, engine oil rings are leaking.  If all else is good.  Perform compression test of each cylinder recording measurement, then add two small squirt of engine oil into each cylinder as you perform a second round, complete both dry then wet compression test on each cylinder before proceeding.  IF any cylinder shows a more significant increase in compression then it would indicate that cylinder's oil rings are faulty.  This is a major repair.  Keep me posted