Screw vs Reciprocating Compressor Oil: Compare the Duty, Not Just the VG Number
Compare the actual oil circuit,OEM requirement and changeover evidence before treating a matching ISO VG as a substitute.
Matching an ISO VG number is not enough to substitute a screw-compressor oil for a reciprocating-compressor oil. Establish the exact machine, the lubricated circuit and the manufacturer's fluid requirement first. An oil can have the requested viscosity while still lacking the documented suitability needed for that duty. This comparison concerns lubricated industrial air compressors, not every gas compressor, refrigeration system or oil-free machine.
Find out where the oil actually works
In an oil-injected rotary screw compressor, oil participates in the compression process. Atlas Copco's technical explanation describes cooling, lubrication and sealing in that architecture. A buyer must therefore consider the oil circuit, separation and operating conditions together; viewing the fluid only as a bearing lubricant misses part of its job.
For a reciprocating compressor, identify the actual lubrication arrangement instead of assuming every design exposes oil in the same way. Establish whether the proposed fluid is intended for the crankcase, cylinder lubrication or another specified location. ExxonMobil's Rarus 800 product information discusses severe-duty reciprocating service and discharge-system deposits for its own products. It explains why duty matters; it does not approve a MOS oil, prescribe a MOS service interval or make the two suppliers' grades equivalent.
This first distinction prevents a misleading purchasing question. “Will VG 46 fit?” is incomplete until the buyer knows what equipment and oil location that request represents. A nameplate, manual instruction and current-oil record are more useful starting documents than a generic compressor photograph.
Read the offered families without merging them
The current MOS catalogue separately lists mineral L-DAH screw-compressor oil in ISO VG 32/46 and L-DAB reciprocating-compressor oil. These are distinct offered families. Their presence in one catalogue is not a cross-approval or permission to substitute one for the other.
An ISO viscosity classification describes a viscosity range on a stated measurement basis; it does not describe the complete additive system, deposit tendency, oxidation behaviour or compatibility with every seal and existing lubricant. The ASTM D2422 classification scope is a useful reference for that limited role. Keep measured viscosity and method attached to the exact offered grade.
Do not transfer a numerical row from one grade to another simply because a product title lists a wider range. Request the current grade-specific TDS and lot COA where relevant. The separate vacuum-pump viscosity guide remains the owner of pump-specific selection; its equipment examples are not compressor approvals.
Compare the missing evidence, not marketing adjectives
| Qualification question | Oil-injected screw duty | Reciprocating duty | Evidence to request |
|---|---|---|---|
| Where is the fluid used? | Identify the injected-airend and connected oil circuit | Identify the actual crankcase or cylinder arrangement | Exact model and manual fluid instruction |
| Which operating conditions matter? | Record thermal duty, separation behaviour and contamination exposure | Record duty and the relevant hot or deposit-sensitive locations | Operating history and manufacturer limits |
| Is the quoted grade suitable? | Check the named fluid against that screw unit's requirement | Check the named fluid against that reciprocating unit's requirement | Current grade TDS and the required approval or supplier confirmation |
| Can the current oil be replaced? | Assess existing-fluid chemistry and the machine's changeover requirements | Assess existing-fluid chemistry and the specified lubrication location | A documented machine-specific transition decision |
The table is a purchasing comparison framework, not a claim that all screw machines share one requirement or that all reciprocating machines need another universal viscosity. If a supplier cannot connect an offered grade to the actual requirement, keep the offer technically unqualified rather than choosing the closest catalogue number.
Treat a changeover as a separate decision
Record the exact current lubricant, not just “synthetic” or “mineral.” A broad base-oil description does not establish miscibility with the proposed fluid or compatibility with residual oil, elastomers and the machine's components. ExxonMobil's screw-compressor technical guide describes product-dependent compatibility and flushing considerations. Its instructions and oil names cannot be copied into a generic MOS changeover procedure.
Ask the OEM and lubricant supplier whether a transition is permitted and what preparation, inspection and monitoring it requires. Do not prescribe an improvised flush, mixing ratio, temperature or running period from this article. Any proposed service interval must follow the particular equipment, fluid and operating conditions, rather than an interval advertised for an unrelated product.
If existing deposits, overheating, abnormal carryover or poor oil condition are the reason for replacement, retain the service and condition records. A new oil quotation does not identify the underlying cause. Resolve the operating problem and obtain a controlled maintenance decision before using an oil change as a presumed repair.
Build a quotation that can be technically accepted
For L-DAH or L-DAB, provide the make, full model, compressor architecture, oil location, required grade or approval, current fluid, operating conditions and maintenance objective. Ask for the exact offered-grade documentation and a written account of any unresolved suitability or transition question. Include quantity, packaging and destination.