I have no idea what this may have been used for - (?)

A regular quart jar and a lid off of a 5 gallon can. Mitch

As Mitch said, the jar is a typical food product jar like for mayonnaise. If it were a glass container designed for oil, it would have a line embossed in the glass with "Fill to line" lettering. Probably was used in a workshop as a makeshift oil dispenser.

I believe it was used for brake fluid.

and would still be very useful for brake fluid !!