Are transmission fluid types interchangeable

[hilux4x4x3]

For an auto transmission, is WS transmission fluid better to put in for auto running type 4 trans fluid. Would it be an upgrade of quality,or is it not advisable to change from type 4 to WS fluid?

[Gipsy]

Edit make sure you flush all the old fluid out, especially if you don't know what it is!
Not sure why but in the 2012 Toyota Hilux owner's manual it says to use only Toyota atf type IV and NOT ws in the 4 speed transmission so it depends what you have! Some people here use Valvoline maxlife since it's manufacturers say it is compatible with Toyota type IV and ws. Maxlife is full-synthetic.

ToyotaWS is made by Exxon/Mobil and is a semi synthetic base fluid, with a proprietary additive package. ... There are countless Aisin Warner transmissions in Toyota vehicles that have eclipsed 200k miles with the factory fill of WS fluid. WS Fluid

The Toyota ATF is really good stuff, and if you go to any of the Toyota 4WD forums (Pradopoint, Lcool), there's a lot of discussion about the merits of the factory oil and the alternatives. Over the 100,000km cycle, it's likely the hardest working transmission will be in the big 4WDs.8 Apr 2016
2002 Toyota WS ATF for Japan

2002 Toyota WS ATF for USA
In 2002, Toyota releases the Genuine ATF WS (World Standard) Fluid Specification (JWS3324 or NWS9638).[16][17] This fluid is not backward compatible with previous Toyota fluids or Dexron fluids. Almost all 2004 and above Toyota vehicles with automatic transmissions use the WS fluid specification. This fluid absorbs moisture and should never be reused. Always use fluid from an unopened container. See Toyota bulletin T-SB-003-11

Toyota asserts that under normal usage, the fluid has a 160,000 km (100,000 miles) maintenance interval for inspection only; ATF-WS does not require any flushing or changing during the life of the vehicle; however, an inspection of the maintenance schedule of many Toyota vehicles reveals that there is a 100,000 km (60,000 miles) replacement interval if the vehicle is used on short trips, city driving, hauling heavy loads, trailer towing, or using a car-top hauler. Check your own maintenance schedule for your vehicle's requirements.

This fluid is not backward compatible with any previous Toyota recommended fluid.

This fluid was first used in the following transmissions:

2004 Toyota P112 Hybrid Transaxle[18]
2006 AB60E 6-speed transmission
2006 U660E 6-speed transaxle
2007 AA80E 8-speed automatic (World's first 8-speed light duty transmission)8[19]
2017 AGA0E 10-Speed Transmission[20]

[martynvella]

I run penrite full synthetic in my 4 speed on the recomendation of penrite. Doesnt need to be completely flushed but the more of the old fluid you can remove the better, the oroginal fluid is completely compatible with it.