[I]f in this the moſt conſummat act of his fidelity and ripeneſſe, no years, no induſtry, no former proof of his abilities can bring him to that ſtate of maturity, as not to be ſtill miſtruſted and ſuſpected, unleſſe he carry all his conſiderat diligence, all his midnight watchings, and expence of Palladian oyl, to the haſty view of an unleaſur'd licencer, perhaps much his younger, perhaps far his inferiour in judgement, …

The reference is to olive oil. In Greek mythology, Athena is regarded as having created the first olive tree.