1. Engine oil wants to be > 80C, principally to evaporate any moisture (engine isn't a closed system) - but there's a thermostat in that loop anyway
2. ATF is a very different oil, and it's a much more closed system
3. When doing an ATF change, the gearbox oil hits >32C very quickly, even though at this point the engine oil cooler isn't doing anything (because the thermo is closed and it's bypassing the oil/air cooler)
4. The original combined cooler doesn't actually have engine & gb oils transferring heat between each other - IMVHO it's just convenient packaging to have them running parallel
5. Keeping the ATF cooler helps prolong its life, which in turn looks after the gearbox more.
6. That's it's temperature IN to the cooler that's interesting only (I had to be taught that one!)
All that aside, whenever I'm putting together a new cooling system, I always fit a few of these to monitor how it's all going and check it's working as I planned with no hotspots - https://uk.rs-online.com/web/p/temperat ... s/9064609/ They might be handy for you too.
Finally if (when!!) I put a new ATF cooler in, I think I'd fit one with a good core capacity like you have, but then probably also fit one of these - http://www.mocal.co.uk/products-oilstats.html (The OT/2 one).
