Oil viscosity

The flowability of the oil, that is, its viscosity, must meet the requirements for use in this engine. Two criteria must be kept in mind:

The oil must not be too viscous, since the starter must be able to turn over a cold engine, and the places in the engine that receive oil must be lubricated immediately after a cold start.

The oil must not be too thin, because at high temperatures and engine speeds the lubricating film may tear.

The chart shows Audi/Volkswagen recommendations regarding oil viscosity:
A – all-season oils meeting standard 50101 requirements;
B – oils with good anti-friction properties meeting standard 500 00 requirements;
C – summer oils meeting API SF or SG requirements.

The temperature values given should be understood as long-term. Short-term fluctuations play no role.

SAE classes
The American Society of Automotive Engineers classifies oils by their viscosity.

Single-Grade Oils
Single-grade engine oils range from thin winter oils SAE 5W, 10W and 15W, through the intermediate SAE 20W/20, to thicker summer oils SAE 30, 40 and 50.

Single-grade oil used to be the least expensive engine oil. For proper engine lubrication, a viscous or thin single-grade oil had to be filled according to the time of year. Single-grade oil is now practically impossible to find at filling stations or supermarkets, but it is still often used in vehicle fleets. For the Audi 80, it is suitable (and this is the manufacturer's position) only as a temporary solution in an emergency.

Multi-Grade Oils
The production of today's multi-grade oil is more complex, so multi-grade oil is correspondingly more expensive than single-grade oil. As a viscosity index improver additive, it contains long molecular chains that swell when heated and shrink again when cooled. In this way, the oil can adapt "elastically" to temperature changes and cover several viscosity grades. SAE 15W-50 oil corresponds to viscosity grade 15W at –15 °C, and to grade 50 at 100 °C.

A problem with multi-grade oils based on mineral oils is that the viscosity-improving molecular chains break down over time, and the oil then becomes less resistant to temperature effects. For this reason, Audi does not permit the use of multi-grade oils SAE 10W-30 and 10W-40 in its vehicles during warm weather.