89 supra non-turbo oil pressure question

[ThaPassion8]

My 89 supra non-turbo is constantly losing oil...but i cannot find any visible leaks, when i check oil it is clean, there is no smoke or odor (like if it was burning) and the oil pressure gauge goes from just above 0 marker when the car comes to a stop to just below 40 lb/in marker when i'm on the hwy going about 70mph...that was right after i had an oil change...the next day it stayed right above the 0 marker between the marker right above 0....i have to add oil constantly! I checked for leaks ...NONE

what is going on??