Fist post here after a long time lurking and absorbing info.
Around two years ago I bought my first automatic car; soon after that I developed an unhealthy obsession with regards to the maintainance of the gearbox. I'm sure you're aware that in Britain, automatics are quite rare. I was the first in my family to own one, despite all of the advice I had received warning me about the reliability. (Truth be told the car I own has awful problems with regards to the clutch and dual-mass flywheel on manual models, so i thought the automatic option was the safer choice)
Either way the gearbox I have is the first generation Aisin TF-80 with the metal valve body cover. It's a very popular box used in loads of US cars made by Volvo, GM and some fords I believe.
I bought the car with 57k miles on it and decided to do a drain and refill to inspect the condition of the oil. To my horror the oil was jet black on this first change so I did multiple flushes with Mobil ATF 3309 which seemed like the best solution at the time.
Fast forward to today and the car is now at 80k miles and after reading many forum posts I have decided to fit an external spin on filter.
This article talking about Volvo's implementation is what convinced me, particularly the last sentence "It could have saved Volvo and us consumers a lot of money to simply have a externally replaceable oil filter on the transmission" (https://www.ipdusa.com/techtips/10176/volvo-tf80-awf21-transmission-issues )
The gearbox in my car has no external cooler, instead relying on a metal an style heat exchanger with radiator water therefore the filter mod was a little more challenging than just splicing into the cooler lines that I have seen many done on this forum.
This is the sandwich plate installed in between the original cooler and the gearbox with the two hoses running to my remote filter head.
Here is the filter head mounted in a cavity in the front bumper:
When I added the filter head I started the engine and allowed the gearbox to pump out 2 litres of its own fluid before I shut it down and refilled the gearbox with the same volume that was pumped out inn addition to the volume of the cooler lines ( I prefilled the filter)
Unfortunately, the oil that came out was black again:
The filter I'm using is a Fleetguard HF7554 which claims to have a beta=2 of 7 microns, which is pretty fine (opinions please) (https://catalog.cumminsfiltration.com/catalog/partsearch.do?&reqCmd=PartInformation&partNumber=HF7554&PartType=Hydraulic,%20Spin-On&Part=-&criteria=part&_locale=en)
Unfortunately, the mount I chose has a M16x1.5 thread which somewhat limits the choice of filter for this application.
My main questions are:
1. Will this setup actually benefit my gearbox; or have I wasted a load of money
2. Is this a good choice of filter to use? (alternative filter that will fit is the MANN W712/45, https://catalog.mann-filter.com/EU/eng/catalog/MANN-FILTER%20Katalog%20Europa/Hydraulic%20Transmission%20Filter/W%20712~45;jsessionid=2630C42D4E6108B08A80D31C0279DAE8)
3. With the current "7 micron" filter, how long should i leave it before replacing it initially? I'd imagine that cleaning up the filth that's in the box may plug it up pretty quickly?
4. Could I use a much larger and even finer filter for this purpose such as the Fleetguard FF5871 which has 98.5% filtration at 4 microns
5. Should I ditch the use of a hydraulic filter and just use a normal ~20 micron engine oil filter as there is no need for such fine filtration on a trans?
Thanks for reading and any comments will be welcome 
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