The Castair FG4000 is a 5-gallon container of full synthetic lubricant formulated specifically for Castair rotary screw air compressors. This is the factory-recommended lubricant for Castair rotary screw units, rated for a 1-year or 4,000-hour service life under normal operating conditions. If you run a Castair rotary screw compressor, this is the oil it was designed to run on.
Why Rotary Screw Compressors Need the Right Oil
Rotary screw compressors operate differently from piston compressors. The oil in a screw compressor does more than lubricate — it seals the compression chamber, carries away heat, and is continuously cycled through an oil separator before the compressed air leaves the unit. Using the wrong oil type causes foaming, varnish buildup on internal surfaces, premature separator failure, and oil carryover into your air system.
The FG4000 is a full synthetic formulation designed to handle the high operating temperatures and continuous-duty cycles that rotary screw compressors run at. It resists varnish and carbon deposit formation, stays stable at elevated temperatures, and does not foam under the continuous agitation of screw operation.
Key Specifications
- Part Number: FG4000
- Size: 5 gallons
- Type: Full synthetic
- Application: Castair rotary screw air compressors
- Service Life: 1 year or 4,000 hours (whichever comes first)
Available Sizes
- GA4000 — 1 gallon (topping off or single-fill service)
- FG4000 — 5 gallons (full drain-and-fill service, shop stock)
The 1-gallon GA4000 makes sense for topping up oil level between service intervals. The 5-gallon FG4000 is the right quantity for a full oil change on most Castair rotary screw units and for shops that want to keep a service supply on hand without repeated ordering.
Castair Oil Lineup — Piston vs. Rotary Screw
Castair makes oil for both compressor types, and they are not interchangeable:
- CAST-LUBE (QT5W30 / GA5W30) — Semi-synthetic, for piston compressors. 4-month or 400-hour rating.
- CAST-LUBE (QT10W30 / GA10W30) — Full synthetic, for piston compressors. 8-month or 800-hour rating.
- FG4000 / GA4000 — Full synthetic, for rotary screw compressors only. 1-year or 4,000-hour rating.
Do not use piston compressor oil in a rotary screw unit. Piston compressor oil will foam under the continuous oil cycling and higher operating temperatures of screw machines, causing separator failure and oil carryover into the air system.
If Your Rotary Screw Has Varnish Buildup
Castair also offers Varnasolv rotary varnish cleaner for units that have accumulated varnish from incorrect oil or extended service intervals. Procedure: mix Varnasolv at 10%, run 50 hours, drain oil while warm, then change oil and separator filters. Do not skip the separator filter change — varnish-contaminated separators restrict airflow and cause oil carryover even after the oil is clean.
When Does a Shop Need This
- Annual oil service on a Castair rotary screw compressor
- Oil change after running contaminated or incorrect oil
- Stocking shop supply for a Castair rotary screw maintenance program
- Initial fill after rotary screw pump service or rebuild
The 4,000-hour rating translates to roughly 1-2 years of shop use depending on daily run time. Set a calendar reminder and stick to the interval — extended oil service on a rotary screw is the most common cause of premature separator and bearing failure.
Why Buy From OEM Diagnostic Tools?
OEM Diagnostic Tools has supplied professional shop equipment and components since 2016. We stock Castair lubricants so you can get the factory-recommended oil without going through distributor channels. Free shipping on qualifying orders, free tech support at 866-217-0063, and full manufacturer warranty. Call us if you need to confirm the correct oil for your specific Castair rotary screw model.
Call 866-217-0063 for quick answers and help!
