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base oil, base oil applications, base oil comparison, base oil composition, Group I, Group II, Sinopec Online

Group I vs Group II Base Oils: Key Differences and Applications

By Sinopec Online Technical Team

Introduction

Group I and Group II base oils are not interchangeable on a one-for-one basis. Processing routes change the molecular mix, which alters viscosity index, volatility, oxidation behaviour and solvency. This article explains the practical differences and how to select the right group for a given duty.

Chemical structure and processing

  • Group I: solvent-refined paraffinic stocks with a mix of paraffins, naphthenes and aromatics. Lower saturation and higher sulphur than Group II.
  • Group II: hydrocracked and hydrotreated paraffinic stocks. Higher saturation, very low sulphur and fewer aromatics, which improves oxidation stability and volatility control.

Typical property ranges

Ranges are indicative and for selection guidance only.

Property Group I Group II
Saturates and aromatics Lower saturates, higher aromatics Over 90% saturates, minimal aromatics
Sulphur Higher Very low
Oxidation stability Lower Higher
Viscosity index Moderate Higher
Volatility Higher Lower
Solvency Good for additives and dyes Lower than Group I
Cold flow Moderate Better at like KV100

Representative grades and uses

Base stock Typical KV100 Common uses
Group I 150N ~5 to 6 cSt Industrial oils needing solvency, process oils, certain greases
Group I 500N ~10 to 12 cSt Gear and marine blends where film thickness is needed
Group II 150N ~4 to 5 cSt Hydraulic, turbine and engine oils with improved oxidation control
Group II 500N ~10 to 11 cSt Higher VI industrial and engine oil blends with lower volatility

Test methods that matter

  • Viscosity index: ASTM D2270 for base stock comparison.
  • Volatility: Noack ASTM D5800 to manage oil consumption.
  • Oxidation stability: ASTM D943 and bench oxidation for trend comparison.
  • Cold cranking and pumpability of finished oils: CCS ASTM D5293 and MRV ASTM D4684.
  • Pour point: ASTM D97 or ASTM D5949.

Formulation notes

  • Solvency and additive response: Group I carries certain additives and dyes more readily. When moving to Group II, adjust treat rates or co-base components to maintain response and seal behaviour.
  • Volatility and VI: Group II helps reduce Noack and can lower the demand for VI improvers in multigrade engine oils.
  • Oxidation control: Group II supports longer drain intervals and higher temperature duty with the right additive system.
  • Cost and supply: Blend Group I and Group II to balance solvency, performance and availability.

Applications and use cases

  • Engine oils and automotive lubricants: Group II is preferred for modern specifications due to oxidation stability and volatility control.
  • Industrial lubricants and greases: Group I retains value where solvency improves additive handling or grease structure.
  • Process and transformer oils: Group I used where solvency and compatibility are important, subject to product specification.

Choosing the right base oil

  1. Define the performance standard and viscosity grade window.
  2. Check volatility and oxidation targets against duty cycle and temperature.
  3. Verify low-temperature limits for the operating environment.
  4. Confirm compatibility and seal performance with the additive system.
  5. Optimise cost and supply by blending Group I and Group II as needed.

Cross-References

Relevant Base Oil Categories

Published by Sinopec Online Technical Team
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