Provided is a process for making a diphenylamine-functionalized poly-α-olefin. The process has the following step: reacting an amount of a poly-α-olefin having a terminal double bond with a diphenylamine in the presence of a catalyst. The poly-α-olefin has a terminal double bond having a number average molecular weight of 120 to 600. The ratio of poly-α-olefin to diphenylamine is 1.0:1.0 to 10.0:1.0. Provided is also a diphenylamine-functionalized poly-α-olefin product.