Tough one. On the back of the bottles, look at the API "donut". If one has SN Plus and one has SN, then, technically, it's a different formula and shouldn't be mixed. If cautious anyway. Doubt it would create any problems though. Oil "quality" should be similar.
Also, tell us the numbers stamped on the bottom or back, and even the copyright date on the label, but I think the left one is the new bottle-label design.
If you keep them, check the seals on the cap to make sure no tampering/re-filling, ya know.