Here's a study from 2017 that shows shear rates in different parts of an engine, Shear rates in engines and implications for lubricant design.Is there some documentation you have about the cam being the area of highest shear? The rotating speeds are fairly slow since each individual lobe is effectively half engine speed; all of the other white papers that have been posted show that the piston skirt/ring pack are magnitudes higher since they are traveling upwards of of 4000+ fpm in some cases.
Here's a table showing shear rates at the cam lobe for a bucket style tappet. I haven't seen any data for roller tappets.
Shear rates at the pistons were high as well in this study, but not quite as high as at the cams. Shear rates in the journal bearings were in the range of 10^6, similar to shear rates for the HTHS measurement.