Thinner oils are being used these days for three reasons: they save fuel in test engines, the viscosity rules have changed because modern injection systems allow lower start-up temperatures and manufacturers (e.g. Honda, Toyota, Ford, GM, Mazda ) are recommending thinner grades. We must give credit to the OEM (original equipment manufacturers) who over the years have produced better and better metallurgy with tighter and tighter tolerances that go through micro-polishing processes. The better finishing of parts has allowed for better lubrication even at a lower viscosity such as SAE 5W-20 or 0W-20 helping with pumping and winding losses without sacrificing robustness and HTHS (high temperature high shear) viscosity as much.
As far as energy conserving 30 weight multigrades go, a big factor in fuel economy is temporary polymer shear. These polymers are additives known as viscosity index improvers (or modifiers) but also pour point depressants. Polymers are plastics dissolved in oil to provide multi-viscosity characteristics.
As oil is forced between a bearing and journal, many polymers have a tendency to align with each other. When this happens, viscosity drops. Then when the oil progresses through the bearing, the polymer molecules entangle again and viscosity returns to normal. This phenomenon is referred to as temporary shear and is measured by the HTHS test.
Oil formulators rely on (temporary) polymer shear to pass the fuel economy test for resource conserving xW-30 multigrade oils. The HTHS viscosity is therefore a lower number (compared to other more robust 30 weights).
HTHS viscosity has become the single, most important measure of motor oil and you can tell how much an oil has wear protection and fuel economy from the HTHS viscosity alone. Note that most modern gasoline engines don’t require too high HTHS viscosity to protect against wear and you can benefit from fuel economy of small HTHS viscosities without causing significantly more wear than with larger HTHS viscosites (in normal driving conditions).
High quality and very polar friction modifiers like organic molybdenum and boron or certain esters also play a big part in fuel economy. High levels of friction modifiers may negatively impact the performance of detergents and dispersants since they all compete on the same (limited) surface. Just the reason why HDEO are wet clutch compatible and also make for a good slow acting/safe engine flush/cleaner.
We think that these oils represent a very careful compromise between fuel economy vs. protection and long drain intervals vs. slipperiness/lubricity. A motor oil is only as good as its weakest link. Balance is of the essence in this case!
Formulating wear resistant thin oils is a challenging task for the manufacturers especially now when the best last resort protection mix -zinc, phosphorus (anti-ware additives) and moly (extreme-pressure additive)- is lower and lower. A good abrasion resistance can be obtained as long as most of the (high) shear resistance is based on a high quality base oil (e.g. POE, PAO) and not on polymer viscosity improvers.
Besides the fuel economy and minimum power losses, these high quality oils offer very good start-up viscosity and good pumping and flow attributes helping with the growing conscience of start-up wear and arctic capable injection systems. They reduce warm-up time transferring heat better but also reduce operational temperatures over thicker oils acting as a better coolant (lower internal friction).
The ACEA defines a Fuel Economy lubricant as a lubricant in compliance with ACEA C1, C2, A5/B5, A1/B1 standards (C-catalyst compatible, A-gasoline engines, B-Diesel).
The API and ILSAC following categories apply to 0,5,10W(winter) multigrades – for gasoline engines only: API SN – RESOURCE CONSERVING and ILSAC GF-5.
These lubricants are mostly suited for service of late model naturally aspirated gasoline engines (low torque at low RPM per cylinder) that are built with tight clearances. (e.g. USDM, JDM)
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