Not storing left-over photopolymer resin back with pristine resin in its original shipping can seems to be a common recommendation.
What is the best practice here? Store in a separate bottle and pour this "once around the block" resin first for the next print?
I suggest the following handling of resins, some basic stuff first:
Now, what to do to get the used resin back to the cycle? Any resin that has been exosed to air and light, such as having been in the vat is best considered to be B-Quality. You can use it to cast greeblies or bits (aka disposal by curing), as one would do with leftover casting (2-component) resin, but that is a waste.
So, let's look at some better ways: first of all re-botteling the resin. We need to take in mind, that the quality of our resin will further degrade the longer it stays exposed to light (and to a lesser degree: air), so we need to handle the resin in a way that allows us to eliminate exposition to either. For this, it would be best to keep an empty resin bottle at hand and label it as the leftover bottle. To fill this bottle, you should use a Jig to keep the vat in a position that it pours into the bottle. You might want to use a funnel in some cases!
Now, we know how to get the stuff back into the (B-quality) bottle. But how to make sure it has the best quality we can? As you notice, many of the jigs involve a funnel. This funnel is used in conjunction with a filter to remove larger particles. The finer the filter, the better. Coffee filters manage to snatch particles down to about 10 to 15 micrometers. It is equivalent to about Grade 4 laboratory filter paper. However, laboratory filter paper of grades 1,2,3 or 602h would allow to catch particles of even lower size, as the mesh gets even smaller, but might clog faster. Tea filters on the other hand have worse filtration ability and should be avoided.
To get the best out of it, use a filtration stack, that starts with a metal mesh filter before going through a rough and a fine filter to get out any chunks and large particles that would clog the fine filter.
It would be best to have this process run in the dark, so mounting the dripping and filtration stack in a box might first sound like overkill, but if you go through a large amount of resin (for example by running several printers) it might be an investment that can save a considerable amount of resin in the long run. However, if you run so many printers, you also might run them continually with the same resin colors and just refill them as needed and only filter if there had been a print failure.
Store your bottles in a closed cupboard. It would be best if this cupboard is ventilated through a filtration unit and then outside. It should also keep a steady temperature above about 10 °C to prevent clumping. Just follow the storage manual for the normal resin actually.
Now comes the tricky part: re-using the resin. While technically the filtered and re-bottled resin should be almost as good as new, it would be best to make sure that we mix it with some virgin resin to make sure we have enough photoinitiator in the resin. For this, I would suggest mixing the recycled resin with between a sixth and half of the fresh stuff. Mix the two well to make sure you get the best possible. Make sure it's the same type and color of the used resin, best even from the same original batch.
Use up the re-cycled resin first, as you should consider it's best before date much shorter than on the virgin bottle.
Considerations for storing resin include using a light-tight bottle, preventing stray ultraviolet radiation from prematurely curing the resin.
You'll also note that users will filter the resin through a fine mesh filter. I've seen some videos in which the user pours through coffee filters to remove as many particulates as possible.
Left-over resin that has been carefully filtered is effectively the same as fresh-from-the-bottle material, from a purely technical view, but you can pick up contaminants that bypass the filter.
