C41 flexicolor developer will oxidize in most tanks/jugs after 4-8 weeks, such that it turns very dark color. That's the only solution of the 4 that has a bad tendency to oxidize. The others keep fine (fix, stabilizer, bleach). RA-4 (RA RT) keeps exceedingly well once mixed. even with no special treatment I have a batch of 10L that has done fine for close to a year. C-41 flexicolor developer is much less stable than that, as noted above (And in Z-131 they mention this too).
The flexicolor concentrates keep well, though. Even in opened bottles, as long as they are topped up with argon/nitrogen or some other inert gas, the concetrates for developer seem to last fine, several months at least.
I started out using liquid tetenal 5L blix C41 kit, back when B&H still sold it. It was ok - not bad. I did find the separate bleach/fix flexicolor gave better results overall though. And its much cheaper per roll than any other product.