A lubricating oil suitable for use in .alpha.-olefin compressors and a high-pressure ethylene polymerization process is disclosed. The oil comprises a base fluid and a hindered C.sub.6-C.sub.30 alkyl 4-hydroxybenzoate. The hindered C.sub.6-C.sub.30 alkyl 4-hydroxybenzoate stabilizes the lubricating oil and does not interfere with the high-pressure .alpha.-olefin polymerization.