a similar idea would be to weld in a vertical fluid level "tube". Since water (and oil) seeks its own level, an external length of clear cross link (think PEX style) of tubing that had a bung at the minimum level, and a return bung at the "max" level. If oil were anywhere in-between inside the tank, it would also be within the tube.
Same idea, but no glass involved. And you (or a DIYer) could modify the tank yourself. Not that Ian isn't/won't do an outstanding job at a very reasonable price. But just offering ideas-