Both are rust and oxidation inhibited, but turbine oils are specified far more tightly for oxidation stability, water separation and air release, because turbines run for years on one charge, ingest water and cannot tolerate entrained air in control oil. Hydraulic oils commonly carry anti-wear or extreme-pressure additives that harm water separation. Substituting one for the other is a real technical error, not an equivalent product at a different price.

Steam turbines ingest water through gland seals as a normal condition of operation, so the oil is continuously challenged with it. An oil that forms a stable emulsion carries water to the bearings, where it disrupts the oil film, promotes rust and accelerates oxidation of the oil itself. Good demulsibility lets the water drop out in the reservoir where it can be drained or removed by a purifier, which is how the charge survives for years.

Varnish is a hard, lacquer-like deposit formed from oxidation and micro-dieseling by-products. These are soluble in hot oil and come out of solution as the machine cools, depositing on the coolest and tightest surfaces, which are typically servo and control valves. The result is sticking valves, slow response and unit trips, often with the oil still passing conventional condition tests. It is one of the most common causes of turbine reliability problems.

Detected principally by membrane patch colorimetry, which measures the colour of insoluble material captured on a filter patch and gives warning well before deposits cause a trip. Routine tests such as viscosity and acid number often look acceptable while varnish potential is rising, so patch testing is the specific tool. Control is by dedicated varnish-removal filtration using electrostatic, ion-exchange or depth media systems fitted as kidney-loop equipment.

Follow the turbine manufacturer's specification. ISO VG 32 and 46 are the most common for steam and gas turbines, with VG 68 and higher used where gearing forms part of the same lubrication system and needs more film strength. Do not adjust grade to compensate for another problem: viscosity is set by bearing design, speed and operating temperature, and changing it alters the bearing film the machine was designed around.

Many years, and often the design life of the machine, provided contamination and degradation are controlled. That expectation is the reason oxidation stability dominates the specification. Life is set by condition rather than calendar: oxidation measured by acid number and RPVOT, water and particulate contamination, air release performance and varnish potential. A charge is replaced when trending shows the additive system is exhausted, not on a fixed schedule.

Regular sampling from a consistent live point, tested for viscosity, acid number, water content, particle count, RPVOT or an equivalent remaining-life measure, and varnish potential by patch colorimetry. Trend the results rather than judging single samples against limits, since the trend gives the warning. Combine this with reservoir inspection, filter differential pressure monitoring and attention to any change in control valve response.