Its very, very old, and a collectors item. Save it, or sell it.
It is probably from the 1970's...many on here would have used cans just like that 'back in the day'!
They used that viscosity designation just to show that the oil would meet the requirements of different 'W' viscosities - it was a 10W-40, but it also met the requirements of a 20W-40 if that visc. was needed.