Re: What causes low oil pressure ????
Oil pressure is a measure of resistance not flow and flow is equally important so bear this in mind when moving to thicker oils.
Thicker oils will give more oil pressure.
Thicker oils handle temperatures better but so do proper synthetic oils where non petroleum basestocks are used.
If you want the best of both world and this is possible you can use a proper 10w-40 or 10w-50 synthetic and this will give you both protection and oil pressure as it doesn't thin down after 2000 miles.
The problem with "hydrocracked" (petroleum based) oils is that they start to thin straight away so what started life as a 10w-60 will be a 10w-30 within a short space of time unlike a synthetic oil which will stay in grade for up to 9000 miles or more. Basically it's the use that makes the difference.
The technical term is "shear stability" and only true synthetics are "shear stable" because the basestocks are more thermally stable to start with and need little or no VI Improvers to prop them up. These important additives are polymers that expand with heat to thicken the oil when hot, unfortunately they are also the weakness in an oil and "shear" causing the oil to lose viscosity.
It's complicated but hopefully this little article explains:
SAE has a test called HT/HS (High Temperature/High Shear) if an oil shears back too much on this high temperature test, it cannot be sold as a multi-grade oil. In fact, the test results from this test are very helpful in indicating the quality of the oil.
The higher the HT/HS number the better because this indicates less shearing. Petroleum oils tend to have low HT/HS numbers which barely meet the standards set by SAE. Because petroleum oils are made with light weight basestocks to begin with, they tend to burn off easily in high temperature conditions which causes deposit formation and oil consumption.
As a result of excessive oil burning and susceptibility to shearing (as well as other factors) petroleum oils must be changed more frequently than synthetics.
Not all multi-viscosity oils shear back so easily. True synthetic oils (PAO’s and Esters) contain basically no waxy contamination to cause crystallization and oil thickening at cold temperatures. In addition, synthetic basestocks do not thin out very much as temperatures increase. So, pour point depressants are unnecessary and higher viscosity basestock fluids can be used which will still meet the "W" requirements for pumpability.
Hence, little or no VI improver additive would need to be used to meet the sae 30, 40 or 50 classification while still meeting 0W or 5W requirements.
The end result is that very little shearing occurs within true synthetic oils because they are not "propped up" with viscosity index improvers. There simply is no place to shear back to. In fact, this is easy to prove by just comparing synthetic and petroleum oils of the same grade.
Synthetics will generally have significantly higher HT/HS numbers. Of course, the obvious result is that your oil remains "in grade" for a much longer period of time for better engine protection and longer oil life.