Originally Posted By: zeng
A sheared oil (and by extension, a fuel-diluted oil) is still offering protection, albeit at reduced protection level as a result of reduced oil film thickness consequential upon reduced operating viscosity.
However main bearings and con-rod bearings might predominantly still stays in hydro-dynamic lubrication regimes offering complete protection from abrasion/adhesion wear mechanisms.
Having said this, in the valve train systems and cylinder liner/piston ring tribo-pairs, there is increased levels of unfavourable boundary lubrication and mixed lubrication regimes ...... which are mitigated by essentially functioning anti- wear additive package.
The same cannot be said of gear-train systems, however.
So I understand better.
If I am using a 5w40 and it shears to a 30w, then isn't it still offering the same level of protection as a 30w?
If my vehicle manufacturer suggests I should use a Xw40, then aren't they factoring in that Xw40 oils are going to shear?