Trade names change; base oils do not. How the four base oils in rotary screw compressor oil behave in Vietnamese conditions, why they cannot be mixed, and a table of which switches need a drain and which a full flush.
Two suppliers can both quote "synthetic compressor oil" and be describing products that must never share a sump. Purchasing asks for a product name; a maintenance engineer should ask for the base oil first, because the base oil settles three questions: how long the oil survives at the discharge temperature, whether the new fill is miscible with what is left in the machine, and what has to happen to the separator, filters and seals at changeover. This article covers the four base oils found in oil-flooded rotary screw compressors and ends with a changeover table you can use in the compressor room.
A compressor oil is roughly 85–95 % base oil by volume; the rest is the additive package (antioxidant, anti-wear, rust inhibitor, anti-foam, demulsifier). The base oil sets most of the physical behavior: oxidation stability at high temperature, volatility, viscosity index, how well it dissolves deposits, and whether it is miscible with other oils. Additives fine-tune the remainder. A lubricant manufacturer can rename a product, change the packaging or switch distributors, but the TDS normally states the base oil, and that is the first line to read. If the TDS does not say, send an oil sample for FTIR before you decide anything.
Mineral base oil is distilled and refined from crude. Severely hydroprocessed grades (hydrotreated, hydrocracked) carry less sulfur and fewer aromatics and resist oxidation better than older solvent-refined stocks, but they are still mineral oils and remain miscible with other mineral oils. The upside: low cost, good additive solvency, friendly to seals and to the paint and coatings inside the machine, and available anywhere. The downside is that the oxidation rate climbs steeply with discharge temperature: oxidation products settle as varnish on the separator, cooler and valves, then thicken into sludge. The reference interval for a good mineral oil is 4,000 hours per the TDS (CPI-9370-46), extended only on the strength of oil analysis; a machine running hotter than normal per its manual, or in a humid room, will come in lower.
Polyalphaolefin (PAO) is a hydrocarbon synthesized from ethylene, so its molecules are uniform and carry no aromatic impurities or sulfur. Against a mineral oil of the same viscosity grade, PAO is less volatile, has a higher viscosity index and a lower pour point, and oxidizes far more slowly at the same discharge temperature. Being a hydrocarbon, it is miscible with mineral oil, which makes mineral-to-PAO the most straightforward upgrade to a synthetic. Its weaknesses are poor solvency for additives and deposits, and a tendency to make seals shrink slightly; for that reason nearly every PAO compressor oil is blended with a proportion of ester to keep the additives in solution, hold seals at the right swell and dissolve light deposits. On a TDS these products appear as "PAO", "PAO/ester" or "synthetic hydrocarbon". Reference interval: 8,000 hours per the CPI brochure at 82 °C discharge temperature (CPI-4601), to be confirmed by oil analysis on each machine.
Esters (diesters, polyol esters) are synthesized from acids and alcohols. The polar molecule dissolves additives and deposits very effectively, gives a high flash point and good heat resistance. Straight ester compressor oils are rarer than PAO/ester blends, but ester turns up as a component in many synthetics, in cleaning oils and in NSF H1 registered oils for food plants. Two things to know when a machine goes onto an ester-rich oil. First, it will lift old varnish and deposits during the first few hundred hours, so the oil filter and separator element can clog early. Second, ester can hydrolyze when water and heat are both present, forming acids; a compressor drawing humid air or draining condensate irregularly needs water content and acid number (TAN) tracked in oil analysis. Ester can also soften certain paints and seal materials, so ask the oil supplier about the machine's seal materials before changing over.
Polyalkylene glycol (PAG, also called polyglycol) is a synthetic base oil with a very high viscosity index and very high thermal stability. Its defining property is that its own oxidation products stay dissolved in the oil, so PAG forms almost no varnish or sludge; that is why PAG or PAG/ester gets chosen for compressors running hotter than normal in humid rooms. In exchange, PAG brings three constraints. It is not miscible with mineral oil or PAO: a mixed fill separates into layers, forms deposits and clogs filters. It is hygroscopic, so drums must stay sealed and the machine must drain condensate consistently. And it can soften some coatings inside the separator vessel and some seals, so materials have to be confirmed before the switch. Plants running PAG should therefore keep dedicated pumps, funnels and containers and label the machine clearly to prevent a wrong top-up. The drain interval follows the TDS of the specific product and routine oil analysis; Hop Tien does not quote one figure for every machine.
| Criterion | Mineral | PAO / PAO-ester | Ester | PAG / PAG-ester |
|---|---|---|---|---|
| Oxidation stability at high discharge temperature | Low | High | High | Very high |
| Varnish tendency | High when hot | Low | Low; also lifts old deposits | Very low |
| Absorbs moisture | No | No | Yes, may hydrolyze | Yes |
| Miscible with mineral, PAO | Yes | Yes | Yes (limited) | No |
| Seals, coatings | Compatible | Needs ester in the blend | Check first | Check first |
| Reference drain interval | 4,000 h per the TDS (CPI-9370-46), extended only with oil analysis | 8,000 h per the CPI brochure at 82 °C discharge (CPI-4601), confirmed by oil analysis | Per the product TDS and oil analysis | Per the product TDS and oil analysis |
The intervals in the table are the oil manufacturer's reference values under its own test conditions; machines running hotter than normal per their manual, in humid rooms or with existing deposits will come in lower. The actual interval for each machine is set by oil analysis.
The first reason is solubility: PAG does not dissolve in hydrocarbons (mineral, PAO), so the mixture separates and forms deposits; ester dissolves in hydrocarbons, but only up to a certain ratio. The second reason gets less attention: two mineral oils, or two PAO oils, from different manufacturers still carry two different additive packages. The anti-foam, demulsifier and antioxidant chemistries can react with each other, leaving the oil foaming, shedding water poorly or losing oxidation resistance sooner than the old oil did. The third reason is records: analysis results from a blend cannot be compared with any product's new-oil baseline, so the plant loses its basis for setting the drain interval. The working rule follows: top up only with the exact product already in the machine; when changing product, even to one on the same base oil, drain completely and replace the filters.
| From → to | What to do | Notes |
|---|---|---|
| Mineral → mineral | Drain hot, drain all low points, replace the oil filter and separator element | On a machine with varnish, sample at 500 hours to see whether deposits are lifting |
| Mineral → PAO / PAO-ester | As above; a machine with deposits should be flushed with a cleaning oil first | The most straightforward upgrade to a synthetic |
| PAO / PAO-ester → mineral | Drain completely, replace the oil filter and separator element | Drain interval shortens; record it clearly on the oil label |
| Mineral or PAO → ester | Drain completely, replace the filter; a machine with deposits should be flushed first | Filters may clog early as ester lifts old deposits; track water and TAN |
| Mineral, PAO, ester → PAG | Full flush: drain, fill with flushing oil or a first charge of PAG, run, drain, fill a second time; replace the oil filter and separator element, check seals and coatings | Residual old oil must be very low; ask the oil supplier for the acceptable limit |
| PAG → mineral, PAO, ester | Full flush as above | Do not shorten the procedure even if little old oil remains |
| PAG → a different PAG | Still drain completely and replace the filters; ask the oil supplier first | Water-soluble and water-insoluble PAGs exist, and the two do not mix well either |
Start with the oil label on the machine door and the maintenance log. If the label gives only a product name, that manufacturer's TDS states the base oil, usually in the description or the specification section. With no label, no log and no TDS, the reliable route is a laboratory sample: infrared spectroscopy (FTIR) distinguishes mineral, PAO, ester and PAG, and it should be done before any new oil goes into a compressor with unknown history. For the other parameters in the report, see Reading an oil analysis report: 6 key parameters; for choosing an oil type by operating conditions, see What oil should a rotary screw compressor run?.
Hop Tien is the authorized distributor of Lubrizol Fluid Engineering (LFE) in Vietnam and carries the CPI compressor oil range across all four base oils: CPI-9370, a sulfur-free hydrocracked mineral oil for machines running within the normal temperature range in their manual; CPI-4601 (PAO) for machines that want long drain intervals; CPI-4600-F (PAO) and CPI-4265 (PAO/ester), both NSF H1 registered, for food plants; CPI-1522 (PAG/POE) for hot, humid machines; plus the CPI COMP CLEAN II cleaning oil used ahead of a base oil changeover on machines that already carry deposits. Hop Tien engineers identify the base oil in the machine from the label, TDS or an oil sample, write a flushing procedure for each compressor following the table above, and sample at 500 hours after the changeover.
PAO and PAO/ester are miscible with mineral oil, but drain completely and replace the filters at changeover anyway, so that oil analysis results stay readable. PAG is not miscible with mineral oil or PAO; a mixed fill separates into layers and clogs the filters.
A small temporary top-up rarely causes an immediate failure, but the two additive packages can react and leave the oil foaming or separating water poorly. Working rule: top up only with the product already in the machine; when you change product, drain completely.
PAG is more thermally stable and forms almost no varnish, because its oxidation products stay dissolved in the oil; that suits compressors running hotter than normal in humid compressor rooms. The trade-offs: PAG absorbs moisture, does not mix with mineral oil or PAO, and seals and coatings must be checked before use.
To PAO or PAO/ester: a complete drain plus new oil filter and separator element is enough if the machine is free of deposits; a machine with deposits should be flushed with a cleaning oil first. To PAG: a full flush with two fills is mandatory, and seals and coatings must be checked.
This article is general guidance; the specific conditions of each machine must be assessed by an engineer.
