Multigrade engine oils have a high viscosity index and therefore differ from single grade engine oils due to a lower temperature dependence of the viscosity. They can therefore be used in a relatively broad temperature range. An engine oil, for example from SAE-grade 5W-40, satisfies the requirements of SAE-grade 5W in terms of cold flow behavior at low temperatures and those of SAE-grade 40 at high operating temperatures so that it can be usable year round even in colder regions (c.f. MB Sheet 224.1 and 224.2).
When manufacturing conventional multigrade oils with a mineral oil basis, viscosity index improvers (macromolecular polymers with a viscosity-increasing effect) are added to suitable base oils. The VI improvers result in a non-Newtonian flow behavior. In other words, the viscosity is not only dependent on the temperature and the pressure, but also on the shear rate in the oil film. Depending on the shear resistance of the engine oil, under high mechanical sheer loads in the engine the polymer chains may break, resulting in a permanent loss of viscosity in the oil (permanent viscosity drop). In addition to the permanent shear loss, a temporary viscosity reduction (reversible change in viscosity) may also occur, depending on the extent of the shear rate in the lubricating gap. For all multigrade oils, therefore, a sufficient shear resistance is required so that even after a relatively long operating period an adequate viscosity is ensured.
Single-grade engine oils such as SAE 10W, SAE 30, etc. only cover one SAE viscosity grade and must be changed depending on the climate zone and time of year. The single-grade engine oils SAE 30 and SAE 40, which are suitable for high thermal loads on engines, make reliable cold starts more difficult or impossible at low outside temperatures and lead to undesirably high viscosity-related friction losses in the warm-up phase. In contrast, the single grade engine oils SAE 10W and SAE 20W-20 which are very suitable for cold starts are not suitable for use at high outside temperatures as the required wear protection cannot be guaranteed.
For the above reasons, it is therefore absolutely necessary to observe and adhere to the limits of the operational fields of different SAE-grades, as specified in the operator's manual and operating instructions and on MB Sheets 224.1/.2.