I don't know if this is the best deal ever, but this weekend AutoZone had a deal where you could get 5 quarts of Mobil 1 and a Mobil 1 oil filter (the 108 is the NA/NB Miata, BTW) for $29 and change. The filter's normally $10, so it seemed like a decent deal.
They also had the same deal on the new stuff (the "Extended Mileage" Mobil 1) for $35 and change.
Chuck