Toilet seat with metal axles in plastic sleeves squeaks like crazy when opened/closed
A teflon based lubricant will be inert with respect to plastics and of course, equally inert to steel and iron products. Teflon based lubricants are used in plumbing applications, specifically in faucet and other water valves, as well as on sliding surfaces for some mechanisms such as 3D printers and low-load CNC devices.

Super-lube is one brand name, although it may not be available in your area as such. A quick search for "teflon grease" results in many returns.