5W 30 ACEA C3 engine oil when 5W 30 ACEA C4 engine oil reccomended

Need to know the engine; petrol or diesel?

C4 is a lower "SAPS" oil (Sulphated Ash, Phosphorous, Sulphur) specifically compatible with exhaust after-treatments such as DEF.

C3 actually has better lubricity properties (because SAPS are a good thing extreme-pressure-wise), but may effect emissions. Chlorine and Zinc and Molybdemumumium [yes I no that ain't write] were popular additives in years past, and they worked GREAT. Problem is, they didn't do much for 3-way catalytic converter health. Oil companies (under "pressure") had to gradually reduce or substitute extreme pressure additives out of their oil formulas to meet new standards.

In fact the "C" in C3 or C4 refers to "catalyst".

So overall, that's a non-answer in a way, and I can't really give a better one, as this question is very technical and may up being "opinion based". What I can say is that C3 in place of C4 in a petrol car would have little to no effect, compared to use in a diesel.

In either case, the longetivity and lubricity in the engine would be improved, with the potential to damage exhast after-treatments increased (C3 in place of C4).

The "tribology" thing has been a weird hobby of mine for four decades. My grandfather worked at Texaco in Beacon NY, in engine labs where they ran V8's at readline for hundreds of continuous hours to evaluate Texaco's engine oils. However, that was when you could by leaded gasoline at the pump with 108 RON.

You can't run stuph like lead and zinc and chlorine and phosphorous and moly- moly- ..... other stuph anymore. It ruins catalysts and harms the environment.

If cost is a significant issue, I'd run the C3. If not, I'd run the C4. Or run what's cheapest "when". The SAPS difference between C3 and C4 is pretty minimal.

All My Opinion Only, with no actual respect for proven fact. Your mil(e)age may very vary...

In Diesel engine C3 oil produces more ash compared to C4, which leads to gathering of ash at the particular filter, thus reducing the filter life. As the particular filter is super expensive I personally would not recommend it, but if you don't care (or if you don't have such a filter in your exhaust) then C3 might have better lubricating properties, as would ACEA A3/B3/B4 type motor oils.