Joth

I have a JDM 98 S/TB, that ive had since 26000km on the clock, i had an oil change done with mobil1 in around 28000 or so, its now at 35000 and i changed oil and filter today (same 15w50 as before).
When I first put in my boost gauge, it always idled (ac. etc all off) around -14 in drive, and around -17-18 in park/neutral. Recently its been idling at less vacuum, with about -11 in drive and -14-15 in park/neutral. I thought maybe the gauge was going bad or something. Today after the oil change the idle went back to -14 in drive and -17-18 in park as it used to.
Whats going on here? Is this a potential ring/sleeve wear problem? Is this a normal compression variation between new oil and oil a bit past the change time?