Abstract: Seals contacted with lubricants, such as poly-.alpha.-olefin lubricants, are made to swell rather than shrink by the use of novel seal-swell agents which are effective even when used in amounts constituting less than 5% of the weight of the lubricant compositions. These seal-swell agents are ester oils composed of molecules corresponding to the formula ROOC--CH.sub.2 CH.sub.2 --[(ROOC)CHCH.sub.2 ].sub.m --C(COOR).sub.2 --[CH.sub.2 CH(COOR)].sub.n --CH.sub.2 CH.sub.2 COOR in which the R's represent alkyl groups of 1-30 carbons, and each of m and n represents zero or a positive integer such that the sum of m and n in a molecule is 0-30.