Originally Posted By: edhackett
The HTHS viscosity is what keeps your bearings operating in the proper lubrication regime. ZDDP does not work in hydrodynamic lubrication, or with soft metals found in bearings. ZDDP only works at high pressures and temperatures when ferrous metals come in contact. HTHS viscosity and ZDDP are addressing different lubrication issues and one does not substitute for the other.
Ed
Ed's response here is my favorite so far.
A HTHS of 3.5 vs 3.4 may not seem like a big difference. That 0.1 mPa.s could be the difference between your bearing's operating in purely hydrodynamic, and boundary conditions. If that was the case, then the ZDDP and other AW chemistry would become active and hopefully protect the engine from wear. Because we are talking about two different lubrication regimes, as Ed said, they address different lubrication issues.
The problem with running a lower HTHS lubricant is that you are likely to increase the amount of exposure to boundary lubrication, in an engine that wasn't designed for that. This then can cause the additive formula to work harder (and therefore wear out faster). This could also result in increased temperatures, higher oxidation rates and other stresses on the oil. Of course this is all dependent on your operating conditions because speed and load of the engine can also play a big role in which lubrication regime you are operating in.
This is being kind of nit-picky on the performance and protection. It's probably not going to kill your engine if you decide to use the HTHS 3.4 oil, but when there are plenty of options out there that meet your required spec.
One could argue that in your specific conditions using the lighter HTHS could provide some fuel economy benefits, as long as you monitored the oil's performance and your operating conditions, to ensure that the increased exposure to boundary conditions didn't have negative effects on things like your oil drain interval length, TBN retention, or increased wear.
The HTHS viscosity is what keeps your bearings operating in the proper lubrication regime. ZDDP does not work in hydrodynamic lubrication, or with soft metals found in bearings. ZDDP only works at high pressures and temperatures when ferrous metals come in contact. HTHS viscosity and ZDDP are addressing different lubrication issues and one does not substitute for the other.
Ed
Ed's response here is my favorite so far.
A HTHS of 3.5 vs 3.4 may not seem like a big difference. That 0.1 mPa.s could be the difference between your bearing's operating in purely hydrodynamic, and boundary conditions. If that was the case, then the ZDDP and other AW chemistry would become active and hopefully protect the engine from wear. Because we are talking about two different lubrication regimes, as Ed said, they address different lubrication issues.
The problem with running a lower HTHS lubricant is that you are likely to increase the amount of exposure to boundary lubrication, in an engine that wasn't designed for that. This then can cause the additive formula to work harder (and therefore wear out faster). This could also result in increased temperatures, higher oxidation rates and other stresses on the oil. Of course this is all dependent on your operating conditions because speed and load of the engine can also play a big role in which lubrication regime you are operating in.
This is being kind of nit-picky on the performance and protection. It's probably not going to kill your engine if you decide to use the HTHS 3.4 oil, but when there are plenty of options out there that meet your required spec.
One could argue that in your specific conditions using the lighter HTHS could provide some fuel economy benefits, as long as you monitored the oil's performance and your operating conditions, to ensure that the increased exposure to boundary conditions didn't have negative effects on things like your oil drain interval length, TBN retention, or increased wear.
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