Oil pressure gauge problems!!!

[Nut Kracker]

I'd still get your oil pressure checked though........

 

[Deuce]

so 1 wire sender earthed via the sender body

I'd advise to attach a extra engine to body earth wire to see if that can assist in the load and fix said issue.
A/C takes a decent amperage to run, which could easily affect the gauge readings given the insufficient earthing abilities.