Oil pressure gauge bouncing cummins ISX

Hello everyone,
Does anyone have an idea what could be the reason my gauge for the oil pressure to bounce between 35 and 65 when i am driving and moat of the rime keeps it on 60-65 ?
Recently I've changed the oil in speedco and put 2gal of lucas for first time,usually always put one but the guy told me that Lucas recomenda two so i did.Changed the oil left speedco and few miles down on the road the gauge went to 60pounds,came back to speedco and had them pour offf 1-2gal of the oil and change the filter again because i thought they've put too much lucas.Took a load CO-OH and down on the road start doing the same thing goes from 35-60-65 without visible reason like most of the time keeps it on 60 then drops down to its normal 35.I use Rotella oil and fleetguard filters,the truck runs fine,no codes on the computer nothing it just doing it whenever it wants.Most of the problems that i was doing research are for lower pressure nothing for higher presure on ISX
Thanks alot

 

How many oil filters the cunmins has?When i came back to speedco they replaced only one!So i am assuming the sensor going bad in that case,right?

 

Thank you,appreciate your help

 

I've bought the sensor which is around $50 and i am gonna replace it tomorrow!Would you tell me where is located or upload some pictures for me please just to make shure i am replacing the right thing
Thank you

 

Appreciate it man!