Assuming the bearing operational temperature = 92 Degree C
The oil viscosity will be:
Mineral VG 100 (VI 100) = 14.6 cSt at 92 degree C.
Synthetic PAO 46 (VI 170) = 10.28 cSt at 92 degree C.
Here we have to consider:
1. We lost 4 cSt, and hopefully it is still acceptable by tribology calculation of the bearing.
2. If We choose synthetic PAO (PolyAlfa Olefin), I don't think so it will helping much. Because PAO is still "Non-Polar" which having same characteristic as Mineral Oil.
3. I prefer to advise to use "Polar" oil which is Synthetic PAG (Poly Alkelene Glycol), then the leakage gas will not easy to dilute to the lubricating oil. But it is little bit expensive
4. Closed loop Vacuum Dehydration system may good to be utilized, but WITHOUT HEATER. Oil tank vacuum extractor can be utilized also.
5. As explained by VALVE, changing oil type and utilizing vacuum dehydration system is not solving the root of the leakage gas. The seal has to be improved.
Good luck, and regards.