bNks334 - The oil to water heat exchanger no longer exists, as already pointed out in this thread. US cars are Oil to air cooled now.
The 128i manual has an oil to water heat exchanger still. In that case, if you had read the thread, I've already conceded that a radiator upgrade might make way for sense for those people (hence BMW's creation of the PPk kit). Their cars do NOT come with the "hot climate" version oil to air cooler.
Those without the oil to water heat exchanger (pretty much all of us) would benefit more from an oil cooler upgrade.
bNks334 - I have an undersized 10row oil cooler from the factory, which is pretty much the only effective way for an N5x engine to shed heat from the engine oil since BMW moved to the "hot climate" oil to air cooler for US production vehicles.
BMW engineers literally said "ok, we can't overburden the cooling system in hot climates by also using it to cool the oil." "Solution: lets further separate the two cooling agents and use a separate oil to air cooler for engine oil."
"hot climate" ? oil to air cooler for US production vehicles ?
All n5x share exactly the same cooling system, in US, EU, AU and everywhere else, period.
I have dealt with 335i specifically, in US, EU and even SA but unfortunately the PDF is still in the works
for you to read it.
Regarding your concern about the coolant temperature "fluctuation" and the oil temperature being somewhat steady, slowly increase or slowly decrease.
The oil temperature sensor and the coolant temperature sensor are located on or around the OFH.
So in theory if these fluid have similar characteristics they will show similar temperature reading but they are not so they do not and is not because how the engine is constructed internally.
Additionally, the thermostats for the oil and coolant are on opposite side, hot vs cool, they open at different temperatures, they have different ranges of fully closed, half open, and fully open, and what the oil system does not have is outlet temperature sensor, what the coolant system has, and is directly linked with the electric water pump that depending on this outlet coolant temperature, plus factoring in RPM and LOAD will either speed up or slow down. No such thing with the oil system and pump.
This thread started with HEATHER ON OR OFF so if you missed it in you damn PDF's the additional coolant radiator is hooked up to the coolant lines from the heater core.
Without insinuating on what is undersized here if there was not need for extra cooling from coolant BMW would not have put additional coolant radiator regardless of its size, never the less hooked up to the heater core.
So inevitable to conclude that using the heater core fan the shred heat works.
Ask BMW why they went this route and until you get an answer stop posting nonsense here.