Go to this page and click on the "Where To Buy" button. It's showing in stock at Grainger outlets in my area.
[URL]https://www.crcindustries.com/api-gl-4-multi-purpose-gear-oil-1-gal/[/URL]
It's a small demand item, so it could be discontinued. Or possibly a batch blending wasn't scheduled soon enough to keep wholesalers and retailers from running out. You could contact CRC and ask them.