Engine oil explained: viscosity, specs and myths

Reading the label: 5W-30

The "5W" is the cold viscosity — how thick the oil is at low temperature (the W means winter). The "30" is the hot viscosity at 100 °C. Thinner cold oils flow faster on a cold start, protecting bearings sooner.

• Lower cold number (0W, 5W): easier cold starts
• Higher hot number (40 vs 30): more protection at high heat and load
• The correct grade for your engine is in the manual

Spec matters more than brand

Viscosity is only half the story. The specification (API for gasoline, ACEA in Europe, plus manufacturer specs like VW 508, BMW LL-04) defines the additive package. Using the wrong spec can starve a turbo or clog a diesel particulate filter.

Common myths

• "Thicker oil stops leaks" — it can mask a leak but starve critical galleries
• "You must change at exactly the interval" — intervals are worst-case; short trips justify sooner
• "Oil additives fix engines" — they rarely fix a real mechanical problem
• "Dark oil means it is dirty" — dark oil is doing its job cleaning; change by interval, not color

Checking the level correctly

• Park on level ground, engine off for 5 minutes
• Pull the dipstick, wipe, reinsert, pull again
• Top up in small amounts between MIN and MAX
• Never overfill — excess oil can damage the engine