SI-916 i-001R8 / SI-915 i-001R9
SI-912 i-001R14 / SI-912-016R19
SI-914-019R19
This SI revises SI-916i-001R7/SI-915i-001R8/SI-912i-001R13/SI-912-016R18/SI-914-019R18 dated 17 July 2025
SERVICE INSTRUCTION
Selection of suitable operating fluids for all ROTAX® 4-stroke Aircraft Engines
ATA System: 12-10-00 Operating fluids
ROTAX Releases Updated Service Instruction on Suitable Operating Fluids
ROTAX has released an updated Service Instruction covering the selection and use of suitable operating fluids for all ROTAX® 4‑stroke aircraft engines, dated 07 April 2026. This revision supersedes the previous July 2025 issue and applies to all certified and ASTM‑compliant ROTAX 912, 914, 915, and 916 engine series.
The update introduces important clarifications and new procedures, particularly around engine oil selection and oil type changes, making it essential reading for owners, operators, and maintenance personnel.
What Is This Service Instruction About?
This Service Instruction defines approved and recommended operating fluids for ROTAX 4‑stroke aircraft engines, covering:
- Engine oils
- Coolants
- Approved fuels, including MOGAS, AVGAS, and ethanol‑blended fuels
- General operating tips affecting engine longevity and reliability
Correct fluid selection is critical to preventing gearbox wear, clutch slipping, contamination, corrosion, and premature engine damage.
Key Change in This Revision: Oil Type Switching Procedure
The primary reason for this update is the introduction of a mandatory procedure when switching between AeroShell Oil Sport Plus 4 and XPS Full Synthetic Aviation Engine Oil — in either direction.
Why This Matters
AeroShell Sport Plus 4 and XPS oils are fundamentally different:
- AeroShell Sport Plus 4 → mineral‑based with gearbox additives
- XPS Aviation Oil → fully synthetic
Mixing these oils can result in reduced lubrication performance and potential engine damage, especially in high‑temperature or turbocharged applications.
Approved Oil Change Procedure (New)
When switching between AeroShell Sport Plus 4 and XPS oil, ROTAX now specifies the following process:
- Drain the oil tank per Maintenance Manual Line (MML) 12‑20‑00
- Add approximately 2 litres of the newly selected oil
- Run the engine briefly
- Drain the oil tank again
- Refill with the selected oil type
No oil system purge is required, and components such as oil coolers or hoses do not need to be opened for this procedure.
Important: After any oil type change, the ROTAX oil tank label must be updated to reflect the correct oil type.
Oil Selection Overview
ROTAX continues to approve oils under two internal standards:
RON 424 Oils
Typically AeroShell Oil Sport Plus 4
Recommended for:
- 912, 914, 915, and selected 916 variants
- Operation on unleaded fuel or AVGAS
Not approved for 916 iSc/iS A and C24 engines
RON 451 Oils
XPS Full Synthetic Aviation Engine Oil
Required or recommended for:
- All 916 engines
- High‑temperature operations
- Turbocharged engines
Strong warning: avoid mixing oil types
Coolant Compatibility Remains Engine‑Specific
The instruction reiterates strict limitations on coolant types:
Conventional ethylene glycol coolants (50/50 mix) are broadly permitted
Waterless propylene glycol coolants (e.g. EVANS®):
- Permitted only for specific 912/914 engines
- Not allowed on any 912 i, 915 i, or 916 series engines
Coolant choice must always be validated at aircraft manufacturer level, as installation and cooling system design play a decisive role.
Fuel Guidance Highlights
ROTAX confirms approval for multiple fuel types, including:
- AVGAS 100LL
- UL91 / UL94
- MOGAS, including E10 (max. 10% ethanol) on approved engines
Key reminders:
- Fuels with more than 10% ethanol are not permitted
- Leaded AVGAS increases oil contamination and may require shorter oil change intervals
- Seasonal fuel blending must be respected to avoid vapour lock
What Operators and AMOs Should Do Now
Review the updated Service Instruction Verify current oil type recorded in engine logbooks Follow the new oil change procedure when switching oil brands Update oil tank identification labels Ensure fluids comply with both engine and airframe approvals
Execution of this Service Instruction must be recorded in the engine logbook.
Need the Right Oil or Advice?
Comet Aviation Supplies stocks approved ROTAX engine oils and consumables and can assist with:
- Correct oil selection
- Oil change guidance
- Compliance clarification for your engine and application
Contact the Comet team if you need guidance.
