Test results. More than you wanted to know?
I measured with a micrometer, six O-ring materials for absorption (swelling) with the Microlubrol 50 CST silicone oil and Meikon silicone O-ring grease. The Microlubrol has the same viscosity as olive oil (50CST), and the grease feels the same as honey (7,000 CST).
* With Microlubrol: After a week, no change in thickness for Buna-N, Viton, rubber, and wetsuit material. For a yellow silicone Inon O-ring, 7% diameter expansion. For a white silicone Meikon O-ring, 12% diameter expansion. For background, measurement of a dry Meikon O-ring diameter over its length showed a 4.5% variation, max-min.
* With Meikon grease: After a week, no change in diameter for the silicone Inon or Meikon O-rings.
The various materials were completely submerged in the oil and grease, not the same condition as the line contact for an O-ring around a housing button. The much higher viscosity of the grease (140 times) probably explains its lack of absorption.
My view is that lubrication around a sliding shaft seal is important; but that's based on high vacuum experience, quite different from submersion in water. Nonetheless, I'll be using the Microlubrol.