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glissade, August 4, 2018 in Maxwell
Put some good 30 wt or 10-40 in it and be happy! Change it often.
I don't have an earlier reference so I leafed through my 1935-53 Motor Manual and noticed that the summer grade (above +32°F) specified for many 1930s makes was SAE 20. Ford/Lincoln/Mercury, Packard, and Studebaker specified SAE 30. I would not expect that an earlier make would have specified an SAE 40 unless ambient temperatures were very hot. For example, the 1938 Dodge owners manual specifies SAE 40 for average daytime temperatures of 90+ °F (ie continuously recurring morning temperature of 70-80°F AND mid-afternoon peak temperature of 100-110°F). Modern oils have much better viscosity indexes so thinner grades do not thin-out in high temperatures like the old oils.
I would try to find an owner's and factory service manual for your car. Until then, I would use a 30-grade heavy duty engine oil. I like Petro Canada Duron SHP 10W-30 but Shell Rotella T4 10W-30 and Chevron Delo 400 10W-30 would also be good alternatives.
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