I have some 1/4 rope, polyester sheath with spectra core. If new it would have a strength of 4000 pounds, but it is 15 years old. It has always been stored indoors, rarely used, and never had more than a few hundred pounds on it. I don't think it has ever gotten wet or dirty. (frankly, I have no idea why I bought it.)
I need to use it to lift my 600 pound sailboat with a crane. Even if it has lost half its strength from old age, it still is plenty strong, but I would feel pretty silly if I lost my boat because I was too cheap to buy some new rope.
I am a climber and the rule of thumb is to replace your nylon rope every 5 years, regardless of how little wear it has. It just isn't worth dying to save $200. But I can't find anything about the life span of spectra (or polyester for that matter).
Tollerowner, Spectra and Dyneema have pretty good records for longevity, as long as there is minimal UV degradation and minimal chafe. From your description, it sounds as though your line should be materially unaffected by those two issues.
Here is an article from Practical Sailor that might provide more insight:
https://www.practical-sailor.com/sails-rigging-deckgear/dyneema-abrasion-test
Hope this helps,
Charles Brennan
I agree with Charles, and as an ex-long time PS customer, them too. If it was stored indoors and at least moderately climate controlled, it should have the strength you need.
If you've got enough, you can double up on the slings and lower the working load. Just a thought. 600# isn't a big load but PRECIOUS!!!