We recently had a compressor with a bearing oil instability issue. Usually one might try adjusting the lube oil temperature to see what effects this might have on the bearing. However, a Consulting Engineer and an on-site Reliability Guru decided to adjust the weight of the oil by adding 68 weight oil into the 32 weight oil. The cut was about 10/90 or 20/80. This did not seem ordinary to me so I have decided to see what other might think. It didn't seem to change anything in the bearing either.

Waylon, I have had a 3-year project on bearing instability in tiny permanent lubed electrical motors to be placed in very varying temp environ. Signature on them are the classic 0.4xRPM giving impressive hi vibe levels compared to size and manufacturer try to fix this with different lube properties at limited progress and very good track record of the 0.4xRPM. Never before seen that many with this the very same vibes. We took a bearing apart and reduced bearing area to increase bearing load and like magic, problem is gone but the manufacturer do not make things "like that". So we are returned to square 1 in this specific part of the magic circus. Good luck with yours. Olov

I'm not sure exactly which part is your question.

Change viscosity is a reasonable approach to confirm or combat oil whirl. But lack of response does not rule out oil whirl.

Regarding mixing of lubricants, I have heard of people doing it. But it needs to be evaluated by the manufacturer: The following quote comes from ASTM D624.
"5.1.5 Make-up oils should normally be of the same type, quality, and manufacturer. Available formulations may change over a period of time. Lubricant incompatibility can arise from mixing differing base stocks and additive packages and should be avoided. When oils must be mixed, testing should be performed in an attempt to determine compatibility. Consideration
should be given to consulting the lubricant supplier(s) and equipment manufacturer prior to mixing oils."

Maybe an analysis of representative (always the tough part) sample of oil at the machine bearings will confirm whether the intended result was initially achieved in terms of viscosity of the mixture. That might go a little way towards answering some questions. (although doesn't answer the long term compatibility questions - getting the oil supplier involved is probably the best way).

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