Unless dictated otherwise by technical data, its generally safe to use the same fluid the system uses as oring lube. Engine oil, hydro, and in the case of fuel, Vaseline/petrolatum.
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Harmon Rocket II -Flying
RV-8 - Flying
Hiperbipe - Sold
PA-20-inspired "family truckster" -in work