UP stopped using #6 in the late 1980's. Their test lab developed their own spec
for a locomotive fuel that was very close to #5. All fuel vendors that suppiled UP had to meet that spec, and samples were taken of each load and tested to make sure they were not sneaking in
road oil or tank bottom sludge.

This product started out as waste lube oil, but that is where the "waste" part ended. It was filtered and processed (re-refined) to remove dirt, water, contaminants, heavy metals, etc. The flash point was adjusted, and it was ready to go. By then it was basically a new product, certainly not in any way resembling the garbage some marginal operators burn.