Originally Posted by ka9mnx
Please go to the ACEA website and download the specs. A5 and A3 oils are not specifically formulated to protect cats. That is where the recent "C" specs come into play. Cat protection was a contrived requirement for the US market only. Europeans used "A" spec oil in cat vehicles for years without any problems. US oil certifiers only "think" high PHos levels "may" be harmful to cats.
From the ACEA:
A/B: Gasoline and Diesel Engine Oils - "High SAPS"
A1/B1 Category is removed with these Oil Sequences.
A3/B3 Stable, stay-in-grade Engine Oil intended for use in Passenger Car & Light Duty Van Gasoline & Diesel Engines
with extended drain intervals where specified by the Engine Manufacturer, and for severe operating conditions as
defined by the Engine Manufacturer.
A3/B4 Stable, stay-in-grade Engine Oil intended for use at extended Drain Intervals in Passenger Car & Light Duty Van
Gasoline & DI Diesel Engines, but also suitable for applications described under A3/B3.
A5/B5 Stable, stay-in-grade Engine Oil intended for use at extended Drain Intervals in Passenger Car & Light Duty Van
Gasoline & Diesel Engines designed to be capable of using Low Viscosity Oils with HTHS Viscosity of 2.9 to 3.5
mPas. These Oils are unsuitable for use in certain Engines - consult vehicle-OEM's owner's manual/handbook in
case of doubt.
C: Catalyst & GPF/DPF compatible Engine Oils for Gasoline & Diesel Engines - "Low SAPS"
Note:
Warning:
These Oils will increase the DPF/GPF and TWC life and maintain the Vehicle's Fuel Economy.
Some of these Categories may be unsuitable for use in certain Engine Types - consult the vehicle-
OEM's owner's manual/handbook in case of doubt.
C1 Stable, stay-in-grade Engine Oil with Lowest SAPS-Level, intended for use as catalyst compatible Oil at extended
Drain Intervals in Vehicles with all Types of modern Aftertreatment Systems and High Performance Passenger
Car & Light Duty Van Gasoline & DI Diesel Engines that are designed to be capable of using Low Viscosity Oils
with a minimum HTHS Viscosity of 2.9 mPas.
C2 Stable, stay-in-grade Engine Oil with Mid SAPS-Level, intended for use as catalyst compatible Oil at extended
Drain Intervals in Vehicles with all Types of modern Aftertreatment Systems and High Performance Passenger
Car & Light Duty Van Gasoline & DI Diesel Engines that are designed to be capable of using Low Viscosity Oils
with a minimum HTHS Viscosity of 2.9 mPas.
C3 Stable, stay-in-grade Engine Oil with Mid SAPS-Level, intended for use as catalyst compatible Oil at extended
Drain Intervals in Vehicles with all Types of modern Aftertreatment Systems and High Performance Passenger
Car & Light Duty Van Gasoline & DI Diesel Engines that are designed to be capable of using Oils with a minimum
HTHS Viscosity of 3.5 mPas.
C4 Stable, stay-in-grade Engine Oil with Low SAPS-Level, intended for use as catalyst compatible Oil at extended
Drain Intervals in Vehicles with all Types of modern Aftertreatment Systems and High Performance Passenger
Car & Light Duty Van Gasoline & DI Diesel Engines that are designed to be capable of using Oils with a minimum
HTHS Viscosity of 3.5 mPas.
C5 Stable, stay-in-grade Engine Oil with Mid SAPS-Level, for further improved Fuel Economy, intended for use as
catalyst compatible Oil at extended Drain Intervals in Vehicles with all Types of modern Aftertreatment Systems
and High Performance Passenger Car & Light Duty Van Gasoline & DI Diesel Engines that are designed to be
capable and OEM-approved for use of Low Viscosity Oils with a minimum HTHS Viscosity of 2.6 mPas.
ACEA A3 is made for CAT protection, as ALL other ACEA oils. When ACEA was formed CAT's were already in use.
C oils (primarily C3) are designed specifically around DPF, but they do help also with protection of catalytic converters due to lower amount of SAPS compared to A sequence.