I made a specific request to the department manager of my local Walmart recently. Here is what I found out:
1. They have "specials" on Mobil One at different times of the year.
2. They sell both 10-30 and 15-50, but often run out quickly.
3. The special price is usually $3.50 or so per quart during the specials.
I asked specifically about Delvac 1 - the manager was not aware of it, but I tried to make the point that this would be a big seller since so many diesel owners did their own changes and that had a need to change often.
Everyone might consider calling their local Walmart to make the point - might make it national level attention.
Also, get them to promise to stock more and to let everyone know when it is going on special.