Look at the API label for "CI-4". This is the service category for HD diesel engine oil. The major national brands are Chevron Delo, Mobil Delvac, and Shell Rotella, but any house brand will do. The primary criterion is CI-4. The common multi-grade is 15W-40, which is suitable for down to about 15-20 degrees F.

CI-4 oils are not generally recommended for spark ignition engines because the additive combustion byproducts can degrade catalysts, but they can be effective for short term use to free sticky lifters or sticky rings.

I use CI-4 oil in my two vintage non-catalyst cars.

Duke