Described is the use of oleophilic esters of ecologically compatible polycarboxylic acids with oil-soluble and ecologically compatible monofunctional alcohols and/or polyfunctional alcohols which may also be watersoluble as the oil phase or as a component of the oil phase of oil-based or water-based drilling fluids and other fluid drill-hole treatment agents. The oleophilic polycarboxylic acid esters are especially suitable as component of the continuous oil phase of inverted fluids of the W/O type. Here--if rheologically suitable compounds will be employed--they may comprise the at least predominant portion of the continuous oil phase; but also possible is the concomitant use of complex oleophilic polycarboxylic acid esters, which are not or just to a restricted degree fluid in a temperature range of from 0.degree. C. to 20.degree. C., as an additive in fluid oils of the class of the non-polluting oils. Here, suitable oil mixing phases especially are oleophilic alcohols, ethers, respective esters of monocarboxylic acids and fluid oils based on oleophilic carbonic acid diesters.