I have an antique outboard lower unit that specifies SAE 40 Nondetergent. It's from about 1950 so the modern oils didn't exist then. They also didn't have the greatest seals then either. SAE 40 ND isn't the easiest oil to find around here and I'd like some thoughts on options. The lower unit has a friction clutch instead of a shear pin in the lower unit so the oil would need to be compatible with that as well. Any modern energy conserving oil would probably be bad for the clutch. Would HDEO oil maybe work in a gear box?