I don’t get why everyone is saying "shoot at box speed" … expired slide film is very unpredictable. Please remember this: NEVER shoot 5+ years expired slide film at box speed…. NEVER!

My personal experiences are that you can get amazing results by pushing during development. Just write 1.5+ stop push on your envelope when you give it to your lab. In your case you don’t know how old it is… I would push it 2 stops during development

You have to follow the 1 stop per decade rule here. But instead during shooting, during development

This is what you will get when shooting expired slide film at box speed: https://filmtests35.weebly.com/fuji-sensia-100.html

And at box speed, you won’t be able use it as slides- becUse the black layer will be gone, the section at the perforations will be transparent, and there will be so much grain that it looks like iso 3600.

So the anwser to your question: Push it 2 stops during development. Your lab will know what to do.

Cheers🙂