I have a question concerning oil viscosity. I've been living in western central Mexico (8000 ft above sea level) for the past 5 years and in that time I've come across all kinds of climates and altitudes. When I originally came out here, I had a 2003 Ford Explorer with a 4.6 V8 that called for 5W-20 motor oil. Ever since the first day my truck was in Mexico I always had problems with the motor oil in my truck especially when traveling up in altitude which would result in an overheated transmission, you could just feel that the truck had no torque whatsoever... until we started using 15W-40, which as we all know is way heavier then the 5W-20 that it calls for.
My question is, why does this make so much of a difference in the engine? How is it possible that putting a heavier weight oil in your engine improves it to the point that it feels like a completely different car. I feel that the altitude plays a key role here, please correct me if I'm wrong.
P.S. I have tried this on several cars now and I always get the same results. Especially on automatic cars.
A. Car makers like 5W-20 oil for two reasons. First, it flows faster at startup. Since oil drains down after shutdown, engine parts are most susceptible to wear at startup. 5W and even the newer 0W oils get lubrication to those parts faster to prevent wear. Second, lighter weight oils provide less internal friction and that improves gas mileage. Imagine how much energy it would take to pump grease through an engine compared to oil and it makes sense that the engineers want as little internal friction as possible.
On the flip side, a heavier viscosity oil does a better job of sealing piston rings against the cylinder walls, so it can actually improve compression and reduce blow-by. But there's a limit to how much you want to thicken oil. Multi-viscosity oil became multi-viscosity because of viscosity improver (VI) additives. VI is a polymer (plastic-like) additive.
Here's a quick description from an additive manufacturer (Afton Chemical): "When viscosity improvers are added to low viscosity oils, they effectively thicken the oil as temperature increases. This means that the lubricating effect of mineral oils can be extended across a wider temperature range. When creating a viscosity improver, a balance between the thickening efficiency and shear stability of the polymer is important. Higher molecular weight polymers make better thickeners, but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear resistant, but do not improve viscosity as effectively at higher temperatures and have to be used in larger quantities. Polymer additives can also undergo thermal and oxidative degradation, unzipping back to smaller monomers, which reduces their effect. The highest possible degree of thermal and oxidative stability is desirable in addition to the features above."
A few decades ago auto makers recommended 10W-40 oil. You don't see that recommendation anymore. Car makers discovered that the quantity of polymer additives required to get oil up to 40 created other problems. In fact, they found that the VI literally baked onto the piston rings causing them to stick in the lands. And creating the heat of combustion cooked the VI on cylinder heads, creating carbon buildup that caused pre-detonation. As noted earlier, since the VI additive is less shear resistant, they don't stand up to long term use.
In addition to the VI issue, you've got heat dissipation issues at higher altitudes. The thinner air doesn't dissipate heat as well and that affects the oil. Oil must carry heat away from engine parts. If it can't release that heat as quickly the oil runs hotter. As oil heats up, oil pressure drops. And lower oil pressure reduces the quantity of oil flow which results in even more temperature buildup. So you can see why a heavier weight oil is good in those conditions. But there's no free lunch here. You're paying a price on the engine wear side of this story. Use a heavier weight oil and you get more internal friction, lower gas mileage, less oil flow when cold, greater chance of piston ring carbon when you use 15-40. My advice? Install an aux transmission oil cooler to take the load off the radiator. That alone will improve engine cooling. (Find automatic transmission oil coolers, oil cooler line, connectors, O-rings, etc. for your specific car or truck under "Transmission-Automatic" in the catalog. Tom)
My question is, why does this make so much of a difference in the engine? How is it possible that putting a heavier weight oil in your engine improves it to the point that it feels like a completely different car. I feel that the altitude plays a key role here, please correct me if I'm wrong.
P.S. I have tried this on several cars now and I always get the same results. Especially on automatic cars.
A. Car makers like 5W-20 oil for two reasons. First, it flows faster at startup. Since oil drains down after shutdown, engine parts are most susceptible to wear at startup. 5W and even the newer 0W oils get lubrication to those parts faster to prevent wear. Second, lighter weight oils provide less internal friction and that improves gas mileage. Imagine how much energy it would take to pump grease through an engine compared to oil and it makes sense that the engineers want as little internal friction as possible.
On the flip side, a heavier viscosity oil does a better job of sealing piston rings against the cylinder walls, so it can actually improve compression and reduce blow-by. But there's a limit to how much you want to thicken oil. Multi-viscosity oil became multi-viscosity because of viscosity improver (VI) additives. VI is a polymer (plastic-like) additive.
Here's a quick description from an additive manufacturer (Afton Chemical): "When viscosity improvers are added to low viscosity oils, they effectively thicken the oil as temperature increases. This means that the lubricating effect of mineral oils can be extended across a wider temperature range. When creating a viscosity improver, a balance between the thickening efficiency and shear stability of the polymer is important. Higher molecular weight polymers make better thickeners, but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear resistant, but do not improve viscosity as effectively at higher temperatures and have to be used in larger quantities. Polymer additives can also undergo thermal and oxidative degradation, unzipping back to smaller monomers, which reduces their effect. The highest possible degree of thermal and oxidative stability is desirable in addition to the features above."
A few decades ago auto makers recommended 10W-40 oil. You don't see that recommendation anymore. Car makers discovered that the quantity of polymer additives required to get oil up to 40 created other problems. In fact, they found that the VI literally baked onto the piston rings causing them to stick in the lands. And creating the heat of combustion cooked the VI on cylinder heads, creating carbon buildup that caused pre-detonation. As noted earlier, since the VI additive is less shear resistant, they don't stand up to long term use.
In addition to the VI issue, you've got heat dissipation issues at higher altitudes. The thinner air doesn't dissipate heat as well and that affects the oil. Oil must carry heat away from engine parts. If it can't release that heat as quickly the oil runs hotter. As oil heats up, oil pressure drops. And lower oil pressure reduces the quantity of oil flow which results in even more temperature buildup. So you can see why a heavier weight oil is good in those conditions. But there's no free lunch here. You're paying a price on the engine wear side of this story. Use a heavier weight oil and you get more internal friction, lower gas mileage, less oil flow when cold, greater chance of piston ring carbon when you use 15-40. My advice? Install an aux transmission oil cooler to take the load off the radiator. That alone will improve engine cooling. (Find automatic transmission oil coolers, oil cooler line, connectors, O-rings, etc. for your specific car or truck under "Transmission-Automatic" in the catalog. Tom)
