Check your owner's manual for a viscosity recommendation. If you don't have one, I think my 1977 Pontiac owner's manual would offer similar recommendations as your 1968 manual.
Engine Oil and Filter Recommendations
Use only SE engine oil (it would have been MS/SD in 1968)
Change oil each 7,500 miles or 12 months, whichever occurs first except under the following conditions:
Driving in dusty conditions
Short trip operation at freezing temperatures (engine not thoroughly warmed up).
Under these conditions, change oil every 3,000 miles or 3 months, whichever occurs first.
Operation in dust storms may require an immediate oil change.
Replace the oil filter at the first oil change, and every second oil change thereafter, if mileage (7,500 miles) is the determining factor. If time (12 months) is the determining factor, then change the oil filter with every oil change. AC oil filters provide excellent engine protection.
See your Pontiac dealer for advice on the frequency of oil and filter changes under unusual driving conditions.
The above recommendations apply to the first change as well as subsequent oil changes. The oil change interval for your Pontiac engine is based on SE oils and quality oil filters. Oil change intervals longer than those listed above will seriously reduce engine life and may affect Pontiac's obligation under the provisions of the New Vehicle Warranty.
A high quality SE oil was installed in your engine at the factory. It is not necessary to change this factory-installed oil prior to the recommended normal oil change period. However, check the oil level more frequently during the break-in period since higher oil consumption is normal until the piston rings become seated.
Note: Non-detergent oil and other low quality oils are specifically not recommended. Only the use of SE engine oils and proper oil and filter change intervals assure you of continued proper lubrication of your Pontiac engine.
Note: SAE 5W-30 oils are recommended for all seasons in vehicles normally operated in Canada. SAE 5W-20 oils are not recommended for sustained high speed driving.
To help assure good cold and hot starting as well as maximum engine life, fuel economy, and oil economy, select the proper oil viscosity for the temperature range anticipated from the following chart:
You should be using a Heavy Duty Engine Oil (HDEO) rather than a Starburst oil in your engine. Any HDEO (CI-4/SL or CJ-4/SM) has enough ZDDP (actually phosphorus) to protect flat tappet valve trains. Starbust oils have enough phosphorus (up to 800 ppm) to protect OEM-style valve trains but HDEOs have more (usually 1000-1200 ppm). If you're in the southern USA where temperatures are consistently above freezing, you could safely use 15W-40 HDEOs. Otherwise, use a 30-weight HDEO (0W-30, 5W-30, 10W-30). Generally, 0W-30 and 5W-30 HDEOs are synthetics, which means they flow very well in extreme cold but maintain their hot viscosity better than 10W-30 and straight 30 in extreme heat.
There is no need to use a higher hot viscosity (second number in viscosity grade) than necessary as this just wastes power and fuel economy.
SAE 30 appears to be the viscosity grade that many older GM engines were designed to use but 15W-40 will generally be cheaper and more readily available than SAE 30. The Corvair Flat Tappet Oil article recommends using an oil that has the correct viscosity range for normal oil operating temperatures. Since every engine has to start from ambient temperatures, a multi-viscosity (like a SAE 10W-30) will have a lower cold viscosity than a single grade (like a SAE 30). The Motor Oil Basics article also recommends the use of multigrade oils.
As for ZDDP levels recommended by Cam-shield, the Corvair Flat Tappet article suggests that phosphorus amounts over 1400 ppm contribute to increased camshaft wear and engine deposits. Phosphorus levels over 2000 ppm can lead to camshaft spalling. CI-4/SL HDEOs contain all the ZDDP required by flat tappet engines and there is no need to add more. The 15W-40 grades usually contain the most and Chevron Delo 400 15W-40, for example, contains 1360 ppm of phosphorus. Phosphorus is the anti-wear component of ZDDP (see What is Zinc?).
You know, I jest about the old mason jars full of dark teal green cheap oil w/ the funnel lid, but back then, I could smell the oil and know that it had good lubricity stuff in it! Darn stuff smelled like the older 90wt gear lube! Or EOS!!
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