Hi Matt
once you have determined that you do not have a problem with your oil sending unit (they do fail without warning, as mine did last month on a 9 year old vehicle), the next thing to consider could possibly be the length of time on the oil you have in the engine since the last oil change, the ambient temperature, and possibly fuel dilution....fuel dilution is very common in carbureted engines and can usually be seen on the dipstick by sight and/or smell, or a noticeable increase in the oil level on the stick.
Scoutboy's response about the sending unit and the oil viscosity covers a good part of what should be researched, with keeping in mind two things: the 30wt oil which was recommended back when the unit was new was the best there was at the time. Since then, multi-weight oils, such as 15w40 or even 20w50 (as he suggested) are the best to use now as they will give more protection over a wider temperature range. With you living in long beach, CA, either one of those weights would be many times better than the straight 30 weight you are using. Secondly; there was a major change in oil additives in all pcmo applications, namely the lowering of the zddp in the oil as mandated by the epa in 2007. It would be best if you would go to an oil which is rated "sl" instead of "sm or sn" as the newer sm, sn rated oils are not good for flat tappet cam engines.
The sl rated oils are harder to come by, but the "swepco" brand, which is used by many for high quality, and more protection, than even the other sl rated oils. It is the choice of IHPA, and is available through their "lubricants" section.
Some people also use what is commonly referred to as "diesel oil" which will carry the cj-4 rating...it will only have a limit of
<1200> ppm of zddp, whereas the swepco 306 (sl) in the 15w40 as well as the 20w50 contains <1700ppm>
in reviewing your "fleet" of vehicles, you might also consider using an sl rated oil in all of them, except for the tesla.
Hope that helps!