Choosing the right oil for your rally or race car isn’t just about grabbing what’s on offer — the wrong choice can sap performance, wear out parts faster, and even cause failure. Here’s a no-nonsense guide to the essentials.

Engine Oil Basics

Viscosity
Oil thickness is measured using the SAE J300 system. Two numbers are shown on the label:

• First number (before the “W” for winter) = cold start flow.

• Second number = flow at normal running temp.

Example: 5W-30 flows more easily from cold than 10W-30.

Why it matters in motorsport:

• Thinner oil = faster circulation on cold start, less drag on moving parts.

• Thicker oil = stronger protection at extreme heat.

Pit Lane Pro Tip: Switching grades? Don’t go too thin if your engine has variable valve timing — oil pressure changes can upset it.

Gear Oil

Gear oils use the SAE J306 grading system. Example: 75W-140.

• First number = performance at 0°C.

• Second number = performance at 100°C.

GL Ratings:

• GL-4 – best for synchromesh gearboxes.

• GL-5 – designed for high-load applications like dog boxes, hypoid gears, and LSDs.

Pit Lane Pro Tip: In competition, a lower viscosity gear oil can free up a few horsepower — but make sure it still has the load protection your gearbox needs.

Oil Standards

• ACEA (Europe) – A = petrol, B = diesel, C = modern low-SAPS oils for emissions systems. Numbers indicate performance level.

• API (US) – “S” for petrol, “C” for diesel; the second letter shows quality level (later letters = newer spec).

Oil Types

• Mineral – Cheap, but only really for classic engines or running-in.

• Semi-synthetic – Mid-range blend; quality varies between brands.

• Fully synthetic – Best choice for competition; resists heat, reduces wear, and lowers oil consumption.

• Castor-based – Excellent lubrication for certain classic or alcohol-fuel engines, but degrades quickly.

Pit Lane Pro Tip: For most modern competition engines, fully synthetic is the safest bet — even if it costs more upfront.

Oil Flow Rates

Right — so when you see something like 5W vs 10W, that first number is a cold temperature viscosity rating, and it’s measured at very low temperatures using the SAE J300 standard.

It’s not a “linear” scale (i.e., 10 is not exactly double 5), but the difference is still significant in real-world flow.

• A 5W oil has to meet a maximum cold cranking viscosity of 6,600 cP at –30°C.

• A 10W oil has to meet a maximum cold cranking viscosity of 7,000 cP at –25°C.

That means at a given very cold temperature, 5W flows roughly 20–25% easier than 10W — and crucially, it stays pumpable at 5°C lower temperatures.

In practice:

• In UK rally conditions, a 5W oil will circulate through the engine a fraction of a second faster on a frosty morning than a 10W.

• That faster circulation = quicker oil pressure build-up = less metal-to-metal contact during startup.

Pit Lane Pro Tip: Even in motorsport, most engine wear happens in the first 30 seconds after a cold start — pick your “W” grade with your coldest expected start-up temperature in mind, not your warmest.

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Cold Flow Comparison — 5W vs 10W

*Measured under SAE J300 standards

💡 Race Rally Pro Tip: Every °C colder at startup makes oil thicker — so the grade that pumps 5°C colder can be the difference between instant pressure and a dry crank for half a second. That’s why 5W is often preferred if winter or early-morning stage starts are in your calendar.

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