Five categories, one brand

For a petrol engine three things matter: viscosity (SAE), quality grade (ACEA or API) and manufacturer approval. In turbocharged and direct-injection engines heat stability comes first. With start-stop, every restart brings a brief moment of dry friction, so cold flow matters too.

In a diesel, combustion leaves more soot. The oil has to hold it, otherwise deposits build up inside the engine. Vehicles with a particulate filter (DPF) need a low-ash oil: the ash from an ordinary oil stays in the filter and blocks it. In city driving the filter never reaches its self-cleaning temperature. Heavy transport uses a separate class rated for tougher duty.

Something 'similar' does not work in this category. A manual gearbox and an axle need an API GL grade: GL-4 for synchromesh boxes, GL-5 with additives that withstand the high pressure of a hypoid axle. GL-5 does not go into a box that calls for GL-4 — 'stronger' does not mean 'more suitable'. In an automatic it is not SAE but the manufacturer approval that decides (the ATF types); DSG and CVT run on their own separate fluids.

Brake fluid absorbs moisture from the air, and the more water it holds the lower its boiling point. In a hot system the water boils and forms a vapour lock, and the pedal goes to the floor. Water also corrodes the brake hydraulics. It is changed by time, not mileage — usually every two years. The class (DOT 4 or DOT 5.1) and ABS/ESP compatibility come from the service book.

For forklifts, excavators, tractor hydraulics, presses and industrial equipment. The grade is chosen to DIN 51524. HLP with anti-wear additives is the standard choice; HVLP has a higher viscosity index and stays stable across a wide temperature swing. Viscosity is given as an ISO VG number: VG 32, VG 46 or VG 68. That figure is taken from the equipment manual, not guessed.