Reducer and Hardener for PPG single stage questions


I bought my PPG single stage awhile back. My hardener is too old. I bought a quart, it was obscenely priced. I have about 6 quarts of paint left, and about a quart of PPG interior paint that uses the same hardener. Mix is 4:1. If I use up my PPG I'll need more hardener. Does it have to be DCX61? Or would SPI hardener work?

Same question for reducer, can I use SPI?

Does reducer go bad with time?

And when people describe reducer as fast or slow, the low temp one is fast? I have PPG 870 and SPI 860. Heading into winter here I probably won't have the garage much over 70 while actually spraying. Might even drop below that while the fans are on, but once they are off I can get the garage into the 80s even in the cold. If I'm a backyard hack and want the single stage to flow out nice, which reducer should I use?
Sorry, you have to buy their hardener.
Yes, you can use SPI reducer.
Reducer won't go bad if it is tightly capped, but there are limits to everything.
Low Temp = Fast, High Temp = Slow.

Reducer selection is based on a combination of temperature and job size. 860 is too fast for anything but small parts and primer spots. PPG 870 may also be too fast, depending on size of job and surface temps.
There are DCX61 substitutes out there. The brand of paint that gets censored here on the forum sells it. (google DCX61 substitute) Others do as well. I can't guarantee you that they will perform as well though. I think I used a substitute DCX hardener once years ago and it did OK. Been a long time. Like I said no promises on how a sub will perform.

I would use 885 and simply give it longer to flash. Keep the temps in the low 70's though. If it will be colder than that then either use 870 or do a blend of 870 and 885.
Another possible option Matrix MH-005 and MH-006 are substitutes for DU5 and DU6. Those were the preferred activators for Concept. Different mix ratio than DCX61 so refer to the TDS if you use one of those. Again no promises on how they will perform.
I looked at my invoice, I was last charged $261 per quart in March. I don't think it makes sense to try to save a few bucks here, I guess I just have to take it...

Thank you for all the replies.