My information is about 8 years old, so take with a grain of salt.

When I was there, petrol was available (from roadside vendors) in Ollague on the Chilean side. I heard rumors of fuel about halfway between Ollague and Uyuni, but had no need. I also heard there was fuel in San Francisco, but again had no need. I actually made it through to Calama without refueling, but I was definitely pushing my luck at a bit over 400 km, which is basically my range on a KLR. I can't remember whether I was carrying an additional ten liters by that time, but that would have been eminently sensible.

There are mining camps scattered around the Bolivian side, and some probably have fuel to spare. When I got lost (GPS-free forever!) I flagged down mining company trucks from time to time, and I'm confident you would not die out there for lack of fuel. Note that this falls into the category of "easy for me to say."

I got hung up by a strike by the border guards, so had plenty of time to check into accommodations in Ollague. There was an obvious roadhouse (restaurant, bar, hotel, and probably brothel) along the main road near the border gates. The guards eventually relented and let me through, but the place looked no better or worse than anyplace else in particular.

Best to get an early start on the day, and not end up driving those roads after dark as I did.

Hope that's helpful.

Mark