Trans Question

If the synthetic fluid they used is spec'd to meet the Toyota Type T-IV ATF requirements - then you should be OK just to drain and refill with the same synthetic ATF. This is assuming that nothing seems to be wrong with the operation of the car otherwise. As long as it meets the specification - you can run something other than the OEM fluids. Lots of owners already do so with zero issues.

If you want to run the OEM fluid - you'll have to flush the transaxle to get all the sythetic out. OEM Toyota Type T-IV is a conventional oil with very specific friction properties, likely would not play nice with the synthetic that is already in there, additive package wise.

There has been a number of discussion here on the forums to do a completely DIY flush, if you decide to go that route.

If you are going to switch to the OEM fluid - you'll have to flush the fluid to avoid any potential mismatch of the additive package. Some pass on the flush, and just do a drain and refill - as the likelyhood of something not being compatible is pretty remote - but that one remote chance than something goes wrong, could be pretty disasterous.

As for procedure - depends on your comfort level, how much work you want to put into this. If going a DIY route - likely looking to drop the pan first to get the old oil out of the pan and clean off the magenets, clean or replace the filter assembly, then reattach and refill. Then have to remove one of the cooler line hoses (return side) and find some way to hold it into the drain pan or into a bucket while you run the car to pump the remaining old fluid out. You can also opt to supply a transmission shop your oil and have them flush it with their machine - but much faster and easier, though you are likely looking at at $100+ to have them do it. The DIY procedure has been discussed a number of times on this forum and there is a huge amount of information online as well.

As for what fluid is safer? Definitely no issue with running OEM fluids - they were the spec'd fluid when the car came out and was part of the original factory fill. OEM fluid will always be the safer bet. If it is the "best" fluid for use - that's where it gets a little tricky.

It is a conventional oil, so depending on driving conditions - it may oxidize very quickly, taking on a very dark color and burnt odor. Some have to change the fluid every 30K miles, others have gone twice that and the fluid still was fine. In fact, the manufacturer (Toyota) doesn't even list a replacement interval for the transaxle fluid under normal operating conditions - just inspections. Replacement is only mentioned under special operating conditions at 60K mile intervals.

Still - some owners have found they transaxle oil to become very dark and oxidized at a relatively short interval - so they opted to change them more often or switch to a synthetic transaxle fluid. Many synthetic ATF now meet or exceed the specifications put forth by manufacturers on their highly friction modified ATF like Toyota's Type T-IV. So in that sense, they are "safe" to use. You may run across some anecdotal evidence that say not running OEM fluids will damage the car - if they are true or not, hard to say with any certainty. Best thing to do is run what you feel is best - if you want to run OEM, run that fluid - if you want to try synthetic, then run that.

On my two 2009 model year Toyotas - one is running OEM, the other is running Valvoline synthetic - so far, no issues. On my Corolla, I tend to drive that car a little harder than most - right off the bat, I switched to a synthetic ATF from the factory fill. My preferred fill is Redline D4 or Mobil 1 synthetic - both works great in cases where they spec Dexron/Mercon ATFs. They also say it is safe for Toyota T-IV applications, though I find that hard to believe, as its viscosity is significantly different than Toyota Type T-IV.