Hello to all !

I have a small MDF vacuum plenum of 14" x 21".

Actually, I have a small GAST 1.1 CFM, 25" Hg but and with leaks here and there, I cant get more than 5-6" Hg of vaccum
I try to fix my leakeage and can have around 10" for certain job but I think 1.1 CFM is too limit.

So I start looking around for a new pump.

I see many many HVAC Vaccum pump on sale on Kijiji (Craiglist equivalent...). 4-7 CFM range with good inch/Hg rating
But all them run with oil.

My budget is around 200$ and I have many questions ;-)

1 - Do these kind of pump work well for CNC clamping application ?
2 - Do I have to wonder about oil mist ?
3 - Do I have to change oil on a regular basis (HVAC world say : change oil after each use....but that's for HVAC application) ?
4 - Can I use my GAST pump in parrallel (with Y push-in connector) ?
5 - Should I stay with a dry pump ?
6 - The price range is fabulous ! Brand new pump on ebay for 150$ and used one at 200$ but with a brand-new cost of 800$ .... What about those "brand new for so little money" ?

Thanks !