From a summary of available literature, since I am too lazy to dig out the textbooks:
Analine itself (C6H5NH2) is also called aminobenzene or phenylamine.
The aniline point is called the "aniline point temperature," which is the lowest temperature at which equal volumes of aniline (C6H5NH2) and the test oil form a single phase and are micible or 'mixable. The aniline point (AP) correlates roughly with the amount and type of aromatic hydrocarbons in an oil sample. A low AP is indicative of higher aromatics, while a high AP is indicative of lower aromatics content.
The aniline point measurement is considered to be a measure of the degree of
solvency power or
polarity of a base oil. Thus base oils that are rich in ester, ether or aromatic functionalities tend to have
lower aniline points than those simply derived from paraffinic or iso-paraffinic feed stocks. So paraffinics or iso-paraffinics have a higher AP.
API Group I base oils have typical AP values of about 100ºC. This increases for modern Group II and III oils since these base oils have much lower levels of aromaticity and polar compounds in their compositions. Polyalphaolefins (PAO) usually have the highest AP values which are typically about 130ºC for commodity lower viscosity PAOs or significantly higher for the heavier viscosity grades. Formulators of modern hydrocarbon lubricants based on Group II-IV oils sometimes find conventional additives are
less soluble in base oils with high aniline points and to improve their solubility they often include a more polar base oil such as an ester or an alkylated naphthalene at low treat rates to reduce the AP point.
OSPs, or oil soluble polypolyalkylene glycols, are more hydrolytically stable, may provide greater fluid longevity, have low AP's and better solvencies, to a point. That is, the higher viscosity OSP's have less solvency.
Below is a picture of an ISO-460 OSP in my lab made by Dow Chemical of Midland Mi. The bottle is actually clear glass and the OSP is water clear, thick, and sticky.
It is a Group V base oil, has a viscosity of 52-55 cSt@100C, a viscosity index VII of 180-200, a Pour Point of -33F, and a Flash Point of 560F.