Heck no! I've pulled under 100 microns with oil that must have been a year old, unsealed. Once your pump heats up the moisture in the oil goes out the exhaust port. Granted, that was an overnighter . . but 92 microns, held for an hour.
yea, it just takes alot longer with that old wet oil
every manufacture of vacuum pump recommend changing oil after every evac. i do mine every third or fourth evac. and i chage right after evac of a burnout. you wouldn't drive your work truck for a year without changing the oil, would you???
Even in large pull downs with large pumps 17cfm.... your vac is only as good as your oil. You will get that milky look to it the initial pull down. change the oil and the closer you get to 500 microns the clearer the oil will look when you change it. I am refering to large supermarket pull downs but the same should apply for any vac I would think.