C3 is not required for cars without exhaust filters because C3 (low SAPS) oil is designed to protect the filter (and the environment) by reducing the ash content in the exhaust.

This presentation http://www.lubrizol.com/EuropeanEngineOils/UEIL2006LowerSAPS.html has some evidence that it might also be of benefit in keeping the engine itself cleaner.

Edit: clarify

[paul]

As you say pip, i think it even has advantages using it in cars with no DPF,

Quote from lubrizol

Summary
• New emissions legislation has resulted in the widespread
usage of advanced aftertreatment systems
• This resulted in the development of a new generation of
high performance “lower SAPS” engine oil technology
• Increasing end user awareness of the need
to maintain vehicle emissions compliance
• Lower SAPS engine oils are entering the
service fill market
– Whilst these lubricants will be more costly, the value
they deliver in terms of maintaining vehicle emissions
compliance will outweigh the costs many times over

Or it might just turn out that EGR is a poor idea for direct injection engines.

Although, in that presentation they also show exhaust valves similarly coated with deposits so that exonerates or mitigates EGR as the cause (at least for exhaust deposits). The inlet deposits on the other hand have surfaced only since direct injection technology - in that no fuel washes the inlet valves.

In any case, I don't see EGR being dropped any time soon... so a reduction in soot from measures like low SAPS oil seems to be the direction we'll go.

[paul]

I agree pip, i have taken an egr valve apart and it was filthy,i personally would do away with mine
but it would more than likely fail the MOT on emissions. but to me it it illogical to stick anything other than clean filtered air in to an engine.but if you HAVE to have an egr valve, low saps oil would probably keep it cleaner.