more generally speaking are 'higher Dexron numbers' also fulfilling the specifications of the lower number fluids ?
Short answer for Transmissions, Yes.
DEXRON®-VI licensed fluids are fully backward compatible and can be used in all applications covered by earlier GM ATF specifications."
"DEXRON®-VI replaces DEXRON®-III and DEXRON®-III-H in all applications except power steering and manual transmissions
All DEXRON-III licenses expired at the end of 2006, and will not be renewed. Beyond that date, General Motors will only support the use of DEXRON-VI fluids for use in Hydra-Matic transmissions.1 Fluids claiming DEXRON-III type performance continue to be sold under abbreviated names such as Dex/Merc and D/M, however, since the DEXRON-III licensing system no longer exists, these fluids are not regulated by GM in any way.
Do not use DEXRON VI as a power steering fluid. It is marked on the bottle not to be used as power steering fluid. DEXRON III/MECRON would be a better choice for cars where the manual specified DEXRON II, which is no longer on sale.
For my Mitsubishi Eclipse 2002 with only 17000 miles, I decided to buy from the dealer DIAMOND, power steering fluid for about $20 a quart. To get rid from old fluid without aerating the power steering pump is a challenge, the rocedure is long. In short lift up and secure the front of the vehicle. Empty the power steering bottle, by siphoning or other means. Disconnect the return hose and connect with a hose to any container or bottle. Plug the hole. Put the transmission in neutral and turn ignition key off. While a helper is turning the steering wheel all the way left then right, add new fluid in the PS bottle. You have to repeat this procedure many times.
Although this may not apply to all cheap fluids, this is a quote from the Mobil website for their premium synthetic ATF. Sure, it's $10 / quart, but isn't your power steering system and transmission worth it. How many quarts do you need per lifetime of the car anyway?
Mobil 1 Synthetic ATF is a multi-vehicle formula recommended for use in modern high performance automobiles, SUV's, SUT's, vans and other light trucks.
It is recommended by ExxonMobil for use in applications requiring DEXRON® III, Ford MERCON® and MERCON® V performance levels.
Meets or exceeds the off-highway power transmission requirements of Allison C-4.
Recommended by ExxonMobil for use in any power steering unit where a DEXRON® or MERCON® fluid is recommended.
The original DEXRON was made from whale oil, which was banned. The initial replacement fluids were not ideal, and the subsequent formulations were addressing flaws in the earlier ones. So it's fine to put DEXRON VI or DEXRON III in most cars.
The thing to be careful of is mixing formulations. Some Chrysler transmissions in particular call for Chrysler-specific fluid, which must be used.