Gear oil parameters 75W-80

What the parameters mean

Viscosity at 40 °C and 100 °C

Kinematic viscosity in mm²/s. The 100 °C value sets the summer grade: 30 starts at 9.3, 40 at 12.5 and 50 at 16.3 mm²/s.

Viscosity index

How little the viscosity changes with temperature. A higher index means a more stable oil.

TBN

Total base number in mg KOH/g. A reserve for neutralising acids; higher helps with long drain intervals. C3 oils deliberately have a lower TBN because of the DPF filter.

CCS

Apparent viscosity at cold start in mPa·s. The limit is 6200 at −35 °C for 0W, 6600 at −30 °C for 5W and 7000 at −25 °C for 10W. Lower means easier starting.

Pour point

The lowest temperature at which the oil still flows. Lower is better.

Flash point (COC)

The temperature at which vapours ignite, measured in an open cup. Higher usually means lower volatility.

Density

In g/cm³, usually given by manufacturers at 15 °C. Used for checking, not for rating.

HTHS

Viscosity at 150 °C and high shear in mPa·s. Shows bearing protection under load. ACEA A3/B4 and C3 require at least 3.5.

Sulphated ash

Ash content in % by weight. For engines with a DPF filter a low number matters; C3 allows at most 0.8 %.

Noack volatility

How many percent of the oil evaporates per hour at 250 °C. Lower means less oil consumption.