Syn series drying up more quickly

I had good success with my syn using coconut oil or shea butter.

How much? Do you just rub everything down? Or do you inject a little inside first?

I tried a couple things. With coconut oil I would try shoving chunks in, but that was very messy. Then I melted some and dipped my refrigerated syn in, making a Butt sicle. That worked pretty well. Then I got a tub of Shea butter. I melt it and use a shooter to get it in deep. That works best for me. Seems not quite as messy as the coconut. ​​​​​​​ I’ve fallen asleep with a tool in and am still well lubed when I wake up hours later. YMMV.

I've got a silicone butt plug that does this with water based lube.

Something thick and oily, like coconut oil or just plain shortening works much
better and lasts longer. Feels better too.

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@Roland Htg a butt sicle.. :D:D:D:D
that comment is pure quality,made my day.

While the "Syn" series of Aneros is certainly more comfortable while the lube lasts, the lube doesn't seem to last as long. I thought maybe it was me absorbing the water-based stuff more quickly, so I tried oil-based and p-jelly types, but they all seem to dry out at a faster rate with the silicone covered toys.

This is a rather strange occurrence that I have not found to be the case for myself, however, I definitely agree that water based lubes dry out more readily than oil based lubes.

What mixture is best for consideration of "longest lasting" first, with "maximum mobility" second?

Please see my response to @Timelesspa 's thread Yet another lube question. Part of the difficulty in answering your question is that the"longest lasting" and "maximum mobility" lubes may be mutually incompatible, rather you will need to find a lube which optimally balances those two qualities for you.
Very high lubricity lubes such as silicone lubes offer maximal mobility and are also long lasting but they are incompatible with any of the 'SYN' models, so should NOT be used. High viscosity lubes such as shea butter and coconut oil offer long lasting lubrication but at somewhat decreased mobility. Finding a balanced lube you are comfortable with is the challenge.
Personally, I am now employing a mixture of 1/3 shea butter and 2/3 coconut oil mix formed into a 3/4" x 3/8"∅ suppository, finger inserted, then coating my Aneros with 'Slippery Stuff' gel for ease of insertion. Eventually, even the water based gel will dry up but the contraction action of the anal sphincters and the Aneros allows a small amount of the oil lubrication to work its way to the exterior, thus keeping the anal orifice moist and lubricated. Long Anerosessions of 2 plus hours are easily sustained with this regime.
Good Vibes to You !