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 ...
15 is thicker oil and for bigger engines.
Yes, you can mix conventional oil with synthetic oil with no problem.
20w-50 indicates Synthetic-oil
motor oil sl cf 20w-50
Yes, I used it today.
15 is thicker oil and for bigger engines.
if it si 15w40 engine oil
20w-40
ist real oil, from the thickness and weight, it sounds like its for a diesel engine
15W40
John Deere Model B uses 20-20W oil. Some shock forks do also. I have an old Hesston Haybine that uses in the hydraulic system.
Not recommended. Use 10w30.