I don't understand how my resources can swing so rapidly sometimes...
Whenever a fleet replenishes it creates huge demand on oil. If your daily oil was +600 but the fleet took on 1,200 oil that day, you will show -600 for that day.
The other thing is the auto system which prefers to run mostly supply convoys one day, then mostly resource convoys next day. On days it is sending out supplies, your supply production slider will increase to build those needed supplies. But your daily oil will drop a lot because it is convoying much oil to overseas depots. On those same days your resources will not be coming in, and so will be considerably less as to their daily figure.
This bouncing of supplies (and oil going out) versus resources coming in changing constantly on alternate days is one reason why many players use manual control. With proper management supplies going out can be keep at very similar daily, and resources coming in each day at precisely the same as every other day. That then allows for logical trading decisions. Also, bouncing in the supply slider causes proportionate bouncing in the upgrading or reinforcements sliders because - if more IC goes to one slider, then less IC available to other sliders.
But, if you don't like to do manual control of convoys, here are some tips to help smooth out your game:
1) Build smaller fleets
2) Avoid several fleets replenishing same time... only one fleet should replenish at a time.
3) Smooth out your conquest... don't go from an idle army to most units suddenly moving.
4) Don't over trade.
5) Alternately, run every merchant ship you can for a day picking up maximum energy so you are several hundred plus for a day... and trade it ALL for oil. If you have +100 energy you can trade that 100 an infinite number of times provided the clock is stopped. Now, when it goes back to normal, it will calculate all, and maybe you 1000 energy daily in the red (because you did 10 trades giving away 100 energy each). It will nicely draw that deficit from your million plus energy stockpile while you enjoy ample oil.
BE CREATIVE!
BTW, it is highly recommended that players who have armies doing overseas conquests (and have accumulated several convoy routes world wide) learn how to do manual control of convoys if only to save on merchant ships needed. The auto system is doing a Herculean task, but unfortunately is victim to "digital thinking", and invariably needs far more convoys than really needed while slowly doing a worse and worse job so that - over time - more and more resources are not picked up and sit in depots while the conquest suffers from inadequate abundance of supply and oil so that any sudden change in demand of the conquest army invariably results in supply/oil short fall unless player has provided a few hundred more convoys than actually needed.