Castair CAST-LUBE QT5W30 is a 1-quart, 5W-30 multi-grade semi-synthetic oil for piston air compressors. At $11.55 per quart, it is the more economical option in the CAST-LUBE piston oil lineup. It carries a 4-month or 400-hour service rating and a -37°F pour point. The QT5W30 is factory-approved for Castair piston compressors and is the correct oil for shops that follow regular maintenance schedules and want quality oil at the lower price point. For extended drain intervals, step up to the QT10W30 full synthetic at $25.41 with its 800-hour rating.
About CAST-LUBE Semi-Synthetic
The QT5W30 is engineered specifically for piston air compressor pumps. It addresses the specific failure modes of reciprocating compressor lubrication: high-temperature oxidation that breaks down conventional oil between service intervals, foaming from air entrainment that disrupts the oil film on cylinder walls, varnish buildup that sticks ring grooves and deposits on valve seats, and cold-start viscosity failure in unheated shops. The -37°F pour point means this oil flows at cold startup in a Minnesota shop in January without pre-heating — cold-starting a compressor with oil that is too thick for ambient temperature is a primary cause of premature pump wear on cylinder walls and bearings.
Key Specifications
- Part Number: QT5W30
- Size: 1 quart
- Grade: 5W-30 multi-grade semi-synthetic
- Pour point: -37°F
- Service rating: 4 months or 400 hours
- Application: Piston air compressors
- Price: $11.55
Oil Properties
- Semi-synthetic formulation: Synthetic base oil with conventional oil blend for cost-effective performance
- High temperature rated: Maintains viscosity at piston compressor operating temperatures
- Low carbon: Reduces carbon deposits on valve seats and piston crowns between service intervals
- Non-foaming: Anti-foam additives prevent oil foaming from air entrainment in the crankcase
- All-season multi-grade: 5W cold-flow rating at -37°F with 30-weight high-temp protection
- Universal piston application: Compatible with standard piston compressor designs
- Factory approved: Specified by Castair for piston compressor maintenance
CAST-LUBE Full Product Lineup
- QT5W30 — 1 qt, 5W-30 semi-synthetic, 400 hrs / 4 months ($11.55) — this unit
- GA5W30 — 1 gal, 5W-30 semi-synthetic, 400 hrs / 4 months
- QT10W30 — 1 qt, 10W-30 full synthetic, 800 hrs / 8 months ($25.41)
- GA10W30 — 1 gal, 10W-30 full synthetic, 800 hrs / 8 months
- GA4000 / FG4000 — Full synthetic for rotary screw compressors, 4,000 hrs / 1 year (do not use in piston compressors)
Rotary Varnish Cleaner — When to Use It
Castair sells a companion cleaner called Varnasolv. If you are servicing a compressor that has been running conventional motor oil or has visible varnish buildup in the crankcase, the transition protocol is: mix Varnasolv at 10% with the current oil charge, run for 50 hours, then drain warm and change the oil and separator filters. Do not run Varnasolv indefinitely — it is a one-time cleaning treatment, not a lubricant additive. After the cleaning cycle, refill with the appropriate CAST-LUBE grade.
QT5W30 vs. QT10W30 — Choosing the Right Grade
For a shop compressor running moderate hours with consistent maintenance schedules, the QT5W30 does the job at the lower price. Change it every 4 months or 400 hours and you are covered. The step up to QT10W30 makes sense when: your compressor runs 6+ hours per day, you want to service it less frequently (8 months instead of 4), or your shop environment is hot enough that extra thermal stability matters. Both are correct for Castair piston compressors — the difference is service interval, not oil suitability.
Technical Specifications
- Part Number: QT5W30
- Manufacturer: Castair (Cast Air)
- Type: Semi-synthetic piston compressor oil
- Grade: 5W-30 multi-grade
- Size: 1 quart
- Pour point: -37°F
- Service interval: 4 months or 400 hours
- Price: $11.55
What's in the Box
- 1x 1-quart bottle of Castair CAST-LUBE 5W-30 semi-synthetic piston compressor oil
Why Buy From Air Compressor Factory Direct?
Factory-specified oil matters for compressor longevity. Generic 10W-30 motor oil from the auto parts store is not formulated for piston compressor duty — the anti-foam package differs, the thermal stability rating differs, and compatibility with valve seat materials may vary. We stock the full Castair CAST-LUBE lineup so you can maintain your equipment with the correct product. Free shipping on qualifying orders. Call us if you are unsure which grade your compressor requires.
Call 866-217-0063 for quick answers and help!
