▁the ▁resid ual ▁min eral ▁o ils ▁that ▁they ▁may ▁be ▁replacing . ▁ ▁So ▁even ▁though ▁small ▁amounts ▁of ▁the ▁old ▁oil ▁may ▁remain , ▁it ▁won ' t ▁c log ▁the ▁system . ▁▁ ▁Poly ol ester ▁oil ▁is ▁used ▁exclus ively ▁in ▁jet ▁tur bine ▁engines ▁and ▁often ▁used ▁in ▁moving ▁picture ▁film ▁cam eras . ▁ ▁Problem s ▁However , ▁the ▁same ▁properties ▁that ▁make ▁it ▁a ▁good ▁sol vent ▁for ▁o ils ▁also ▁make ▁it ▁a ▁good ▁sol vent ▁of ▁und es ired ▁things ▁left ▁behind ▁during ▁the ▁manufact uring ▁of ▁a ▁unit . ▁ ▁Some ▁of ▁which ▁are : ▁dust , ▁d irt , ▁resid ue ▁from ▁sol der ing , ▁small ▁bits ▁of ▁metal ▁from ▁cutting , ▁and ▁ox id ized ▁metal ▁from ▁the ▁tub ing . ▁The ▁resid ues ▁collected ▁can ▁c log ▁the ▁system ▁filters ▁and ▁cause ▁excess ive ▁wear ▁or ▁damage ▁to ▁critical ▁components ▁such ▁as ▁the ▁van es ▁or ▁val ves . ▁ ▁This ▁aspect ▁makes ▁it ▁more ▁important ▁to ▁check ▁that ▁components ▁are ▁mach ined ▁properly , ▁deb ur red , ▁and ▁clean ed . ▁ ▁Another ▁problem ▁with ▁these ▁o ils ▁is ▁that ▁they ▁are ▁h yg ro scop ic ▁in ▁nature , ▁which ▁means ▁they ▁can ▁abs orb ▁mo ist ure ▁from ▁the ▁atmosphere . ▁Hence , ▁techn icians ▁working ▁with ▁units ▁that ▁have ▁to ▁be ▁opened , ▁should ▁leave ▁the ▁unit ▁open ▁for ▁as ▁little ▁time ▁as ▁required . ▁In ▁addition , ▁PO E ▁o ils ▁typically ▁have