Oil is essential for making any ester,
The ester gives the compound a longer time for de-esterifying & would be effective on the body for a longer time as Sustanon, Deca, Primo ...
also not all esters are the same of course they depends on the number of chained carbons at the atom
water based steroids cannot be esterified as Winstrol, Suspension are not esterified and have a short half life
if somebody tried to esterify Winstrol or other water based steroid then an Oil as sesame or cotton seed oil must exists