You definitely should talk to the owner / manager. Be polite, of course.
Managing a store is not always easy, and it's almost certainly time-consuming in a way that salaried employees have no idea. This may be just an example of a rep coming into the store, talking the price sheet and margin (I guarantee it will be higher than industry average ... that's how they sell it to the retailer ) and he just went "OK, we'll try some". I'd bet there were cases in the salesman's vehicle for ready drop off. Maybe he even walked in with a case and just left it there.
But, in a friendly way, you could give him a "heads up" that this type of oil is known to be damaging to motors and that news outlets do occasionally run stories about it and who is selling it locally. Maybe bring evidence or show him the relevant websites. Frame it like "I like this store and wouldn't want it to get any bad publicity for an innocent mistake".
Should work.
I can see some managers who will want to sell the inventory before not buying more ... the bottom line, I've paid for this already, Yadda Yadda.. That is the wrong solution, and I'm not sure I would cut him much slack if that was the case.
If not, call the local TV station (but give him a reasonable amount of time before you go nasty on him). They LOVE to do stories that make the station out as looking out for viewers. Increases station loyalty and that turns into a better Rate Card, which pays the bills and generates the profits.
