Is a thicker engine oil always better for protection?
No. Thicker oil builds a thicker film only where the engine was designed for it, and it costs friction, pumping work and flow everywhere else. The right viscosity is the one the engine’s clearances and oil system were designed around — then adjusted, within the approvals, for how the car is used.
What viscosity is
Viscosity is a fluid’s resistance to shear. In a plain bearing the journal drags oil into a converging gap and builds pressure — hydrodynamic lubrication. The load the film can carry grows with viscosity and speed, and falls with load and clearance (the Sommerfeld number captures this relationship). The Stribeck curve shows the regimes: at low speed or high load the film thins and metal contacts asperities (boundary lubrication); at higher speed a full film forms and friction becomes purely viscous.
Reading the grade
In an SAE grade such as 5W-30, the first number describes cold behaviour (cranking and pumpability), the second the viscosity at operating temperature (kinematic viscosity at 100 °C, plus a minimum high-temperature high-shear viscosity at 150 °C). Multigrade oils use polymers that limit how much viscosity falls with temperature (the viscosity index). Under very high shear, those polymers align and viscosity drops temporarily — one reason the HTHS value matters for bearings.
Protection vs power
Thicker oil means more viscous drag in every bearing, on every cylinder wall and in the pump: power and efficiency are lost to heat. It also flows more slowly at start-up, when most wear happens, and through small passages. Modern engines with tight clearances, variable valve timing actuators and hydraulic chain tensioners are calibrated for a given viscosity: oil that is too thick can make actuators respond slowly and trigger timing faults, without adding protection.
Thinner oil reduces losses but leaves less margin when the oil is very hot, diluted by fuel or sheared down. That is why the manufacturer specifies a range and an approval, not just a number.
Turbo vs naturally aspirated
A turbocharger shaft spins at very high speed on a film of engine oil and sits next to a turbine fed by exhaust gas. The oil there sees far higher temperatures than in the sump, so thermal and oxidation stability matter as much as viscosity: poor oil cokes in the bearing housing. Direct-injection turbo engines also suffer fuel dilution, which lowers viscosity in service. Turbo engines therefore benefit more from strict adherence to the approval and from shorter intervals under heavy use. Naturally aspirated engines are more tolerant, but high-revving ones depend on a stable film at high piston speeds.
Track and heat
On track, oil temperature rises well above road values, and some owners move to the heavier end of the manufacturer’s allowed range, with an oil cooler where needed. That is a decision within the specification, made with oil temperature data — not a general rule that thicker is safer.