I had a UOA done on Mobil 1 5W-30. Mobil states the viscosity @100c is 10, but my UOA with 6300 miles shows 11.6.
Why would the oil thicken ? I thought it would thin out.
Thanks
Well, first you have to assume that the lab is right, and that the PDS is right. Some of the oils, and subsequently their spec sheets, changed with the move to GF-6/SP.
There are three factors in play that affect viscosity in service:
1. Fuel dilution - Driving down viscosity
2. VII shearing - Driving down viscosity
3. Oxidation - Driving up viscosity
Typically, blenders shoot for balancing #2 and #3 so that the oil remains roughly the same viscosity over its planned service interval. That is, as some of the VII polymers shear, a bit of base oil oxidation compensates for it, balancing it out. Where a wrench gets thrown into this party is with #1, which will drive down overall viscosity regardless and has become more and more prevalent with the introduction of GDI/TGDI.
If you have a port injected engine, fuel dilution is less likely to be an issue and subsequently, it's possible to see a bit of oxidative thickening in service.