Welcome to the Stovebolt forum from Fort Erie, ON!
My Motor manual only go back to 1935 but it should give you some idea of what oil you require. The 1935 crankcase capacity is
5 quarts and the normal oil pressure is
12 psi @ 50 mph. I recommend getting correct owner's and factory service manual for the car. The following viscosities were recommended for 1935:
Above +32°F - SAE 20
Above +10°F - SAE 20W
Above -10°F - SAE 10W
See
YouTube: Motor Oil: "Riding the Film" 1937 Chevrolet Engine Lubrication 11minSince engine oils back then were non-detergent, any modern oil would be a vast improvement. You should be able to use any modern 20-grade oil (0W-20, 5W-20, 10W-20). By definition, all 20-grade oils have the same viscosity range (ie, 6.9 to <9.3 cSt) once warm (ie, 100°C/212°F). All oils become thinner with increasing temperature but multigrades do not thin-out as quickly as monogrades. 0W grades generally require synthetic oils.
For extra antiwear protection and far better engine cleanliness, I would also suggest a 30-grade
heavy duty engine oil (0W-30, 5W-30, 10W-30, 15W-30), preferably with a dual certification (ie, API CK-4/SN). I like
Petro Canada Duron SHP 10W-30 but
Rotella T4 10W-30 and
Chevron Delo 400 SDE 10W-30 would also be good choices.
I would not go with a 40 or 50 grade oil (eg, 15W-40 or 20W-50) because these engines had splash lubrication and were specified with a 20-grade maximum viscosity. Excessive viscosity will cause more oil to relieve back to the sump so less reaches the bearings. See
Corvaiir Oil Guide and
Engine Wear.
While "W" in a grade does stand for "winter", the number associated with the modern "W" in the grade does not denote the temperature at which the cold flow rate is tested. The W-grades are based on maximum viscosity specifications at various temperatures using ASTM D3244 and ASTM D5293 test methods. See
SAE Viscosity Grades for Engine Oils.
Your engine may contain a large amount of sludge. Heavy duty oils can help remove this sludge because the detergents slowly lift sludge from engine surfaces and the dispersants keep them suspended in the oil. The Corvair Oil Guide has the following recommendation:
Bottom Line Recommendation #10:
Forget the myth that you can’t put high detergent oils in older engines or engines that have been using poor quality oil. I do it every day! 50% of this market is API SF or lower, frequently without thermostats. They are full of sludge. Some drain plugs come out looking like a cork, with an inch or so of thick sludge on the end. No matter what the engine, I put in a 10W-30 high detergent CI-4 oil and instruct the customer to come back when it thickens up, or the following week if he doesn’t want to check it himself. Once it no longer thickens up quickly we move on to 15W-40 and add a 1200 mile engine cleaner. At the end of that cycle we move to whatever oil the engine should have.
