Car oil gauge is wonky and unsure why?

If you only experience this after a long stop and go commute, it's heat. Are you cutting corners on oil quality? You need to use an API -SN. Your oil is losing viscosity at high temperature and thinning out. Modern oils are reformulating to increase cat converter performance, but at the expenses of older car performance. The suggestion of 40 weight oil is reasonable but even a thirty weight with a high temp rating should help. You engine is beyond the multi-grade stage.