2001 Montero Sport with 144k miles. A friend of mine borrowed the truck, and got the tranny flushed while he had it. Supposedly, the guy that did the flush put ATF type HK fluid in it. He also told my friend it had a leak.

When I looked into adding some fluid due to the leak, the stick says to use SP-III, so I'm totally confused. Which am I supposed to put in it? HK or SP-III?

 

Mitsubishi transmission fluid ONLY!!! Nothing else, regardless of what designations it has.

 

So, obviously you're rather passionate about it being Mitsubishi-branded only, but does that mean SP-III like the stick shows? It seems to shift just fine with the HK in it, if that's what he really put. The guy that did the flush also mentioned that the PCV valve being stuck could cause weird problems, and after I replaced that yesterday, it is shifting better that it's ever done since I bought it a few months back.

However, I do seem to still have the high-RPM issue in 4th gear that I reported in another post, so not sure about what to do with that.

 

No, I'm not passionate about Mitsubishi branded fluids. Just heard too many stories where transmission was refilled or flushed with non-Mitsu fluid (correct grade) and that's when transmission starts to misbehave. Most of the time doing a flush with actual Mitsubishi fluid (unfortunately this has to be done at the dealer) clears the issues most of the time.
There is a lot of discussion about this online (here an on other forums). It seems that transmission clutches just don't behave the same way with any other fluids. I have no idea why. I'm not a chemist or mechanic, so i do not have a good explanation for this. I'm just another Mitsubishi owner who works on my own vehicles and try to keep them happy.
I've heard similar stories with other makes. Honda is one example where you MUST use branded fluids for certain services even though there are a thousand less expensive choices.

 

OK, so tried to order some of the OEM fluid from a Mitsu dealership, and they cancelled the order. So, I called them up to find out why ("can't ship fluids" is what I'm told), and the parts guy told me that the Hyundai/Kia fluid with a part number of "um010-ch002" is a direct substitution for the OEM fluid. Turns out this dealership handles all three brands (Hyundai/Kia/Mitsu), so I'd think they know what they're talking about.

Just FYI and FWIW...