How Oil Viscosity Affects Engine Starting

Why Increased Oil Viscosity Significantly Increases Starting Current

The fundamental reason is that higher oil viscosity greatly increases the engine’s starting resistance torque.

1. Increased oil viscosity → greater starting resistance torque

When the engine starts, the starter motor must overcome:

• Compression resistance of cylinders

• Friction resistance of moving parts (pistons, bearings, etc.)

• Internal viscous resistance of the engine oil

At low temperatures, oil becomes much thicker (like honey), dramatically increasing the viscous drag inside bearings and between piston rings and cylinder walls.
For example, at 0 °C, 15W‑40 oil has a viscosity of about 1000–1500 mPa·s, while at 100 °C it is only ~14 mPa·s – nearly a 100‑fold difference.

2. Higher resistance torque → starter must produce higher torque

For a DC starter motor (series wound), torque is approximately proportional to armature current:

T∝Φ×IT∝Φ×I

where ΦΦ is the magnetic flux (nearly constant).
Thus, if required torque increases, the current II increases proportionally.

3. Physical process – current surge

• Normal condition (moderate viscosity):
  Torque ≈ 20–40 N·m, current ≈ 200–400 A

• Low temperature, high viscosity (e.g., –10 °C):
  Required torque may rise to 80–150 N·m (several times higher)
  → Starter current may jump to 600–1000 A or more

• Severe case (oil too thick, engine cranks very slowly or stalls):
  Current can approach 1200–1500 A (near locked‑rotor current)

4. Real‑world example

For a small diesel engine at 0 °C:

• With 15W‑40 oil: starting current ≈ 400 A

• With 5W‑30 oil (lower viscosity): starting current ≈ 250 A

Each step up in viscosity grade (e.g., from 5W to 15W) can increase starting current by 30–60% at the same low temperature.

5. Why “significant” rather than “small”?

Because oil viscosity increases non‑linearly as temperature drops and shear rate (engine speed) changes:

• A 10 °C drop can double the viscosity

• Doubled viscosity → roughly doubled resistance torque → doubled starter current

Since starting currents are already hundreds of amperes, doubling them adds several hundred amperes – a severe extra load on the battery and starter.

Conclusion

Higher oil viscosity → higher starting resistance torque → starter needs more torque → armature current increases proportionally → battery discharge current spikes, voltage drops sharply, which may prevent starting or damage the starter and battery.

Therefore, for diesel engines in cold climates, always use a low‑viscosity, multi‑grade oil (e.g., 5W‑30, 0W‑40) according to the expected minimum ambient temperature, and consider using an engine block heater to reduce starting current.