fuel dilution

1) fuel dilution will naturally cause viscosity thinning due to the mixture of a low viscosity fluid (fuel) with a higher viscosity fluid.

2) Fuel in the oil will volatize. Some will be recycled through the combustion chamber and burn off. Some non-volatile components will be left behind as solids. These will tend to thicken the oil.

3) Fuel chemical interaction with the oil will cause increased oxidative thickening. For some oils, this thickening will tend to offset the thinning due to fuel dilution.

4) There are some highly active aromatic hydrocarbon molecules in fuel that do nasty things to some oil and VII molecules, snipping the long chain molecules apart. This seems to happen with some oils that use VIIs and causes what Terry Dyson calls chemical shear or aromatic damage to the oil.

5) Fuel is a solvent, and as such will soften the tribological additive layers on the bearings. This softening or washing causes an increase in additive depletion and increased wear of some bearing metals in some engines.

As a side note, if fuel does indeed cause aromatic damage to the VIIs, that would also subsequently cause a reduction in HT/HS, since HT/HS is primarily adjusted using VIIs in many oils.