I do not use synthetics. The trade off criteria is: for the additional cost is it worth it, does it give you the extra protection for the additional time frame that it make it economical? Lots of discussions go on about that topic.
If you have an oil/hydraulic testing protocol in which you regularly take samples and send them off to a lab so that you can track the results of time versus degradation that might help justify the cost of switching but if not then I would not do it but that is just me.
If you do not switch everything at the same time then you will quickly dilute the synthetic and you will lose any benefit that you might have gained. I know my new Fecon head just took nearly 20% of my reservoir. If you have a 20 gal reservoir or so with synthetic and 5 gallons in 5 different implements after the first implement is installed you have a reservoir with 80% synthetic. After the second implement is hooked up your dilution factor changes again and so on. Very quickly you have mostly normal fluid and very little synthetic left in you main unit.
Think of it this way 5 gal in 5 units and your 20+ gallon reservoir. 25 gallons normal and 20+ gallons synthetic. After several implement swaps you have about 45% synthetic and 55% normal fluid.
That is forgetting the fact that many say that you should never mix fluids and to ensure a complete drain before switching over.