I have had so little luck with olive oil that I no longer try. I do believe that the difference between distilled and tap is worth the price but you do not need reagent grade stuff. The dollar store version works. I might add that I never have tried to clean enough coins at a time that much water gets used. I suppose if you had 1000 absolutely filthy coins that it would make sense to wash them off first in a bucket of tap water but when doing 3 or 4, you can splurge. I do not buy uncleaned lots but only clean things I think could use a touch up..
Just go real slow with the lot, soaking in distilled water and cleaning with a cotton swab (Q-Tip) everyday for a month.