Since we have several diesels on the place I buy Delo 15w40 by the case. It goes in most everything including the 51's 216.....in the summer. I change to 10w30 in the winter. I'm likely overthinking things but I do know how viscous 15w40 is when it gets below 30 degrees. This winter (bragging...yes, yes I am) it started @ 18 degrees and I drove it 15 miles to Lit'l Bro's house. It's still 6v and I don't think it'd have started with Delo, it Barely cranked.
