The two were designed for different groups of vehicles, different duty goals and different test criteria, so they are not 1:1 equivalent standards.

• API CK-4 — heavy-duty diesel engines such as trucks and industrial diesels that run continuously at low rpm under heavy load and high heat · tested for heavy load, soot handling and shear stability
• ACEA A3/B4-23 — passenger cars (A3 = petrol, B4 = light diesel) · tested for cleanliness, stability and oil-film strength

Think of M/T tyres built for rock and mud versus H/T tyres built for tarmac: each excels at something different. These are engine-oil standards from different worlds, with different jobs.

The international lubricants industry doesn't go by cylinder count alone. It looks at four things: GVWR (gross vehicle weight rating), duty cycle (operating hours per day), oil and emission standards, and type of work (towing, long hauls, low rpm under high load).

• Light-Duty, e.g. Toyota 2.4/2.8 GD, Ford 2.0 Bi-Turbo, VW 2.0 TDI, BMW 2.0/3.0 Diesel → ACEA A3/B4 (no DPF), C2/C3 (with DPF), API SP/SN, CI-4 Plus, CJ-4, BMW LL-04, MB 229.51
• Heavy-Duty, e.g. Cummins ISX, Detroit Diesel, Volvo D13, Isuzu/Hino trucks → ACEA E7 (no advanced exhaust after-treatment), E9 (with it), API CI-4/CI-4 Plus, CK-4/FA-4, Cummins CES, Volvo VDS
• Big-engine pickups such as the Ford 6.7 Power Stroke, RAM 6.7 Cummins and Chevy 6.6 Duramax can tow several tonnes and have a high GVWR, so they count as Heavy-Duty — Euro 5 and later should use ACEA E9 or API CK-4

Both are heavy-duty diesel standards for the Euro 5–6 era with DPFs, and both are Mid-SAPS (they don't wear out a DPF quickly the way High-SAPS oils such as CI-4 or E7 do). CK-4 is American (API); E9-21 is European (ACEA).

• They are tested on engines from different sides of the Atlantic: CK-4 on Cummins and Mack, E9 on Mercedes, Volvo and MAN — so the additive formulas differ
• ACEA requires HTHS ≥ 3.5 for every grade and is strict about oil-film stability
• E9-21 tightens oxidation control, piston cleanliness and wear control → suitable for long drain intervals
• CK-4 focuses on high-temperature durability, soot control and shear stability, suiting high-torque American-style engines

1. Base stock — Group 4 PAO, whereas most ordinary API oils use Group 3 · PAO handles heat better, evaporates less and holds its viscosity even when very hot
2. Moly Xec® & Ester Xec® additives plus ACEA A3/B4-23, which requires HTHS > 3.5 — a tougher oil film than standards such as API SP, which focus on fuel economy and lower emissions
3. Built for heat and shear — designed for high-boost tuning and engines that build up a lot of heat, and A3/B4-23 tests sludge protection and piston cleanliness more strictly

For tuned cars (remap or upgraded turbo) or high-performance European cars, Motec® I A3/B4 is the most fitting choice.

• Drag — slightly heavier feel only when pulling away, because the film is thicker; fuel use may rise by about 1–3%
• Hot weather of 35–45°C has almost no effect on engine-oil temperature at full operating temperature (normally around 90–105°C)
• City driving / traffic jams — 5W-30 is more responsive, circulates well and is more economical
• Sustained high rpm / heavy loads / full boost — 5W-40 better supports the load on bearings and ring grooves, reducing the risk of oil-film breakdown

Advice: if the engine runs smoothly on 5W-30 and uses no more than 1 litre per 10,000 km, keep the same grade · if the engine gets noisier when very hot or you often drive long distances, move up to 5W-40

• MB 229.3 (≈ ACEA A3/B4-16) — Group III (VHVI) base stock or partly PAO · drain interval around 15,000–30,000 km
• MB 229.5 (≈ ACEA A3/B4-23) — mainly Group III+ or PAO base stock · at least 1.8% fuel saving · Long Life around 20,000–40,000 km

A real case: a Ford Ranger 2.2 T6 manual was filled with ATF by a garage. After a while the gears became hard to engage, because ATF breaks down very quickly in a manual gearbox.

That's why newer Isuzu models no longer recommend engine oil in manual gearboxes, and most Ford service centres now use MTF-type gear oil. After switching to MITEC® MTF 75W-85W, customers felt a clear difference from the very first fill.