While many assume that similar viscosity means interchangeability, that's a risky misconception. Diesel oils are formulated with nearly double the amount of detergent additives compared to petrol oils. These additives are essential for diesel engines, which generate more soot and microscopic debris due to their combustion process.

However, in a petrol engine, those high detergent levels clash with anti-wear additives. This imbalance weakens the engine's protection against friction and wear. As Lake Speed Jr. highlights, choosing oil based solely on viscosity ignores critical differences in chemical formulation—differences that can make or break your engine's longevity.

Lake Speed Jr. put his theory to the test through comparative lab experiments while at Driven Racing Oil. He tested four oil samples with similar viscosities, two for diesel, two for petrol engines. The results were clear: the diesel oils caused significantly more wear on engine parts.

This increased wear is directly linked to the chemical imbalance in diesel oils when used in petrol engines. The added detergents may seem beneficial, but they reduce the effectiveness of anti-wear agents, leaving vital components exposed. Over time, this means faster degradation, higher repair costs, and potentially total engine failure.

Choosing the wrong oil might feel like a harmless shortcut, but for petrol engines, it's a shortcut to major damage. Stick with oil designed for your engine type—your wallet and car will thank you.