Alternatives to ISO VG 68 Oil and ISO VG 100 Oil

Just google "viscosity chart".

This is one comparative chart: http://www.globalindustrialsolutions.net/vis-chart.php

Similar ones are on other sites.

I also need to add to my previous comment that there are attributes other than viscosity that must be considered if you are changing oil spec. I have had no problem getting hydraulic fluids with the ISO VG designation in various viscosities. Is there a certain need to switch?

Additives are very important to consider since vacuum pumps usually condensate water inside after the suction process.

Aside from that, you should also consider the fact that parts in your pump could be slightly worn out, so you could use a higher viscosity oil to compensate this, but be very careful, because if you over do it, you will burn your motor out.