Calling VIN Masters & Rear End experts

[Pucho]

I had the guys on this forum run my VIN just to see what I have and was happy to learn that I apparently have a 25% LSD. With this info in hand I go to the dealership to get the OEM diff oil. Parts lady asks for my car's year, make & model and tells me it's not an LSD and insisted that only the M series have these according to her techs. I offered my VIN but she said there's no way to tell what diff I have with the VIN number and that she would not be responsible if I bought the wrong type of oil.

Ok fine, so I buy the recommended oil for an open diff and go home. Still not convinced, I put the car up on jacks and spin the wheels. They both turn in the same direction - WTH

So now totally confused, I turn to you guys for some input, suggestions and answers.

1) Can a VIN really tell what original rear diff a car has?
2) Would a LSD oil work just fine on an open diff but not the other way around?
3) Is there any other way to tell what diff I have without opening it up?

I just want to put the correct oil in. Thanks,

Dan