I can't comment on underground coal. But have worked in Iron ore and above ground coal. After the mines get discounts from large companies like Mobil, etc. In Iron ore we used Mobil with some Lubrication Engineer's specialty products. In coal we used Shell mostly except for drag line lubes which were Bel Ray.I wanted to make a side note about mystic in general. Years ago, when u did a lot of work in coal mines and rock quarries mystic was a main stay in oils, and grease. Never seen any more or less wear in pins, engines or transmissions then using any other brand. Price was always decent.
For the same reason many mines use GM trucks.