I have a customer whose last few oil samples from a pressure fed, filtered gearbox on a Z-Mill has had lower viscosity than the oil that was put in the gearbox. The only thing abnormal we can see in the samples is there is a great deal of air entrainment and foaming when collecting the sample. The oil settles out after a couple of hours with the top off the sample bottle. I didn't get any information on the type of oil, etc. yet. I suspect either a leak in the suction line. The sample reports do not show any mixing of two different kinds of oil, therefore I do not think incompatibility of additives is the cause.

My question is this: Does air within the oil have any detrimental effects to viscosity??

Once the air has settled out of the sample it should not effect your viscosity readings. To reduce the possibility of contaminating your sample loosen but do not remove the cap this will allow entrained pressure bubbles to be vented off the sample without floating particles settling in your sample and effecting your particle counts.

A change in viscosity could be caused by oil cracking or the wrong oil being used as a top up lubricant.

Perhaps you are seeing some viscosity shear if the oil contains a VI improver or polymer thickener.

To me, it stands to reason that air bubbles can displace oil and result in an effect equivalent to reduced viscosity (increased potential for metal to metal contact). Particularly in rolling element bearnigs and gearboxes.

I read something similar regarding free water droplets but I can't find the reference. Also I have no direct experience to prove any of this.

If you test the oil while the bubbles are still entrained in the oil it may effect your viscosity results. Most test slates direct to allow the sample to settle so the air comes out and you can do you visual inspection prior to running the instrument tests.

I found a reference. EPRI TR1004384
"... air in its various forms may have an impact on the ability of a lubricant to perform its design function....."

"Air entrainment.... is extremely small bubbles dispersed throughout the bulk of the oil.... Some of the potential effects of air entrainment include: pump cavitation, spongy and erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation, component wear due to reduced lubricant viscosity...micro-deiseling due to ignition of the bubble sheath at the high local temperatures generated by compressed bubbles..."

Thank you all for your input. We are currently trying to find the source of the air leak. The suction lines are under the floor. The pump is seldom turned off, so the tell tale "oil on the floor" is non existant. We are also looking at the possibility of the seal on the pump allowing air to be sucked in. Don't know whose seal it is yet, or what particular kind (mechanical or rubber), but are researching that.
Pete, some of the things you mention from the reference is what we are seeing to some degree.
We have to find and stop the leak.
Again, I appreciate the knowledge you have shared.

Dave