15W40
Yes, you can mix conventional oil with synthetic oil with no problem.
It takes 15 quarts of 15W40 diesel oil I highly recommend Rotella 15W40
Only the weight of the oil, 20w 40 is thiker than the 15w40, 20w 40 is the best oil for the old cars although 15w40 is very easier cold start ...
15w40
15w40(CH4)
10W40 or 15W40
15w40
15W40
12 quarts of 15w4012 quarts of 15w40
15w40
12 quarts 15w40