Consider power washing...
with a flexible lance. There are a lot of safety precautions involved, but I have much practical experience with doing this in a refinery environment. It is common to have something that needs cleaned that you just cannot reach.
This is a 2-person job. Replace the nozzle with a ~10' section (or longer) section of pressure washer hose. Connect the nozzle to the end of the hose with a steel (not malleable) coupling and a short piece (12") of sch 40 steel pipe. The short piece of pipe is to keep the snake from turning around and coming back out and is very important! stuff to hose in through one of the openings, through the baffle and into the part you cannot reach. One person holds the hose and works it around, while the other holds the trigger, ready to release it at ANY sign of trouble. The person holding the hose should were very heavy gloves. The sections you can reach can be cleaned with the normal lance.
You will need to pump the water/oil mix out as you work. Most diaphragm sump pumps are rated for oil. DO NOT use a shop vac (mist = boom).
There is also the option of a tank cleaning nozzle - I do not think you need to go that way.
Please google "pressure washer safety" for more tips. This is a standard industrial method referred to as "flex lancing."
Be careful. It is a piece of cake, if you take precaution against letting the flexible snake out of the tank under pressure. If it comes out of the tanks while powered up, someone will get cut. That is why the short piece of rigid pipe is very important. It cannot leave the tank.
(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")
"Well, I just climb up to them."
by Joe Brown, English rock climber
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