I know what I'll do.
I'll just build x fuel harvester bases.
Then I'll divide 360 days by x.
Create a colony somewhere around 360/x days travel time for the fuel runner, and order it to visit each fuel base in turn, and drop it off at that colony.
(my fuel runner can hold the same amount of fuel that my fuel harvesters harvest in a year (360 days)).
Then I'll build another fuel runner that can hold exactly enough fuel to keep making trips from the drop-off colony to the place where I actually want the fuel uninterrupted.

I've never had my harvesters run out of fuel. Though I tend to create a colony/fuel dump in the system they are operating in, so they head there instead of anyplace outside the system.

This does not work for me. The order 'refuel from target fleet: harvester base' is deleted after the message 'cannot complete orders, not enough fuel'.
The tanker does not wait until it's full
It just grabs as much fuel as there is available, and proceeds to drop it off at the designated point.
If I cycle the orders, this will eventually result in the tanker running out of fuel while transporting it, because it keeps dropping off 90% of it's fuel, but not gaining enough at the fuel base to compensate.

I tried some different orders: I set the conditional order: if fuel tanks full, unload 90% fuel at colony within 4 jumps, combined with the cycled order: refuel from target fleet: fuel base, with an order delay of 1 month (to prevent my increments getting cut short all the time from the cannot complete orders message spam).
This worked swell for a turn. Unfortunatly, the game does not remember order delay setting when cycling moves, so after one 1-month increment I was back to getting an interrupt message every 2 hour increment.

Now I just give my tankers the command to refuel from the bases manually every month, combined with the 'fuel tanks full >> unload 90%' conditional order.
For now it's doable, I only have 2 fuel bases and 2 tankers, but I can see this becoming a major PITA when I will have more bases and tankers running, if I have to keep doing this manually.

Really now, how can I set up a set of repeating / conditional orders so that I do not have to look at my fuel bases / tankers manually every month?

Heh. Another question.
I set the conditional order: "Fuel tanks full, unload fuel at colony and move to Sorium Gas Giant"

It moved to the nearest colony, and unloaded it's tanks, but it is not moving back to a Sorium Gas Giant.
Do I need to establish a colony an the gas giant for that?

All TANKERS are codeing for First Flight with 10% fuel onboard.
Give the Ordert to REFUEL at colony..or send it at nearest orbital Harvester on a agas giant then go ahead with normal orders.