[SAJ]

Glycerine or glycerol (same thing) mixes with water. It absorbs water from the atmosphere and then corrodes iron and steel. In the 60's I had shocks that were filled with old glycerine which had turned brown and thickened. I cannot imagine after 60 more years there are any unseized houdaille shocks using glycerine, unless someone has religiously flushed and renewed the glycerine.
Being water soluble it will not mix with oils, hence the reason to check the contents. Easiest is to get some on a stick and then see if it washes off with water. I have about a dozen old shocks that had glycerine in them. They are all ruined with corrosion now. All are dismantled and I bought 8 Stipe shocks for two cars because the old houdailles were too far gone to fix without making new rotors, and re- sleeving the bodies. Far too much work for me to contemplate until I retire.
I have not heard of castor oil being used. Shocks get very hot on rough roads and castor oil gels when heated and then cooled. That is why racing engines using it had to be drained immediately after a race.
"Air blown " castor oil is a very thick form and used in old fashioned non-hardening gasket cements and modern rubber greases. Another disadvantage in a shock absorber.
Stipe shocks call for synthetic 20 weight shock oil if I recall correctly. Worn shocks can use 90 or 140 weight oil to compensate for wear, in a suck it and see exercise. This helps slow leakage too.
SAJ in NZ