What oil do you guys use? Its 10W-40 for my truck in the TX heat and 5W-30 in winter. Thought some of you gear heads would like some info. If not to educate you, at least to pass the time Viscosity is the measure of the internal friction in a liquid or the resistance to a flow. Because Viscosity changes with temperature and sometimes also with pressure, it is also important that when different fluids are compared that the measurements were conducted under the same temperature and pressure conditions. The multi's have viscosity index improvers. Those are chemical additives that are added to finished lubricants to improve the viscosity index. A lubricant additive, usually a high-molecular-weight polymer, that reduces the tendency of an oil to change viscosity with temperature. Multi-grade oils, which provide effective lubrication over a broad temperature range, usually contain V.I. improvers. While Viscosity Index Improvers can enhance viscosity index (VI), they can break down under shear or over time, resulting in diminished performance. The viscosity index is an arbitrary scale used to show the magnitude of viscosity changes in lubricating oils with changes in temperature. Oils with low VI number such as VI=0 ("zero") have high dependence of viscosity change on temperature. They thicken quickly with decreasing temperature, and thin out quickly with increasing temperature. Oils with high VI number such as VI=200, will still thicken with decreasing temperature but not as rapidly, and also will thin out with increasing temperature, but again not as much as low VI oil. VI number can also be "negative". However, VI does not tell the whole story -- it only reflects the viscosity/temperature relationship between temperatures of 40°C and 100°C. Two lubricants or base oils with the same VI number may perform dramatically different at low temperatures in the -5°C to - 50°C range. some some other info I ran across: No matter what oil you use for any purpose the ideal viscosity that provides the ultimate lubrication, that is TOTAL bearing surface separation, and at MINIMUM power that is consumed by the lubricants viscosity (MINIMUM TEMPERATURE RISE) occurs ONLY at ONE combination of: SPEED LOAD TEMPERATURE. Under ALL other combinations of the three factors, the lubricant is NOT IDEAL. Some lubricants, due to much higher than normal viscosity index, can have more advantageous performance over much wider range of TEMPERATURE, SPEED and LOAD, than others and therefore can be used more universally in wide range of applications. That is why some lubricants such as single grade SAE 30, must be changed to SAE 20 when operating temperature is reduced or to SAE 40 or SAE 50 when the operating temperature is increased. So thicker more viscous oil is needed when engine is operated at higher temperature such as high summer heat. Similarly the proper viscosity depends on LOAD, the higher the load the thicker or higher SAE number is required. So on highly loaded engine designed to used SAE 30 oil under normal operation SAE 40 or SAE 50 should be utilized. Speed however has the opposite effect, when engine designed to run at 2,000 RPM is constantly run at 6,000 RPM but at the same load, the SAE 30 oil should be substituted with SAE 20 oil. Higher operating speed requires thinner or lower viscosity lubricant. It is possible in some applications that the increase in load can be just offset by the increase in speed and then the same oil such as SAE 30 that is just right for NORMAL operation will be also JUST RIGHT for the new HIGH LOAD and HIGH SPEED regime. "Old" truckers are well aware of this from experience, they get much better and longer engine life when running in lower gear up-hill. Extra LOAD is imposed on the engine by climbing uphill (lifting cargo weight against the pull of gravity requires more power therefore the engine LOAD is increased), this can be balanced by running engine at much higher RPM (this requires thinner lubricant). The alternative of running uphill in low gear, that is at slow engine speed and increased load would surely require increase in motor oil viscosity or else almost certain engine damage would result. It would be rather inconvenient to change motor oil before and after every major hill on the Interstate. Therefore changing gears is much more feasible. Thinner motor oils such as 5W-20 or even 0W-20 are becoming more popular these days and are even specified by some OEM's (FORD & HONDA) on new 2001 cars. Although these oils are promoted as "energy conserving" they generally trade a gain of less than 0.1 MPG in Corporate Average Fuel Economy (CAFE) for shorter useful engine life. FORD which has previously designed cars to have 10 year or 150,000 miles life has reduced the mileage life expectation to "beyond 100,000 miles" on vehicles that are operated on SAE 5W-20 Motor Oil. HONDA only claims "useful life" as 7-years or 70,000 miles in EPA certifications for their CIVIC which uses SAE 5W-20 Motor Oil, while the previous model that utilized SAE 5W-30 Motor Oil was certified for 10 year or 100,000 mile durability. Since both HONDA and FORD Warranty their NEW cars for ONLY 3-years or 36,000-miles the reduction in engine life expectancy is not a factor. By contrast Mercedes-Benz recommends use of ONLY Synthetic Motor Oil that is at least SAE 5W-40! This is a recent increase in recommended viscosity from SAE 5W-30. Apparently customer research indicated that engine longevity is more important to typical MB customer than fuel economy.