M1 0W-20 is stocked at many Walmart's (not all), so try to find a big one (and a newer one) in a more affluent area (where customers are likely to have newer cars).
If you are talking about a brand that is not normally stocked in that weight in any Walmart, that is a little harder since they don't have a history of any demand. I would ask someone in Walmart automotive for a phone number to call to discuss this with Walmart management (at a higher level than store level). Once I was able to talk to the guy in charge of ordering all optical products for Walmart at corporate headquarters in Bentonville.