I'm sorry if this question is a DUH! but my quick search didn't give an answer.

I have December 2001 TJ Sportswagon with the 5 speed gearbox which is due for a gearbox flush and charge.

As I understand it from the internet the fluid is fairly specific, Penrite have mineral & Synthetic multi-vehicle fluid that is supposed to be compatible - is this safe to use?

Best bet is to use the Mitsubishi stuff, if you have a dealer nearby. It's generally the same price or often cheaper than Nulon, Penrite, etc. and is effectively guaranteed to be the right stuff. The fluid you mention is probably fine but I'd check price and availability of the OEM stuff first.

No, use genuine mitsi fluid for the trans, those things are meant to be really anal and many members have had issues which have been resolved by using the genuine fluid. Penrite oils are good for the motor, but the trans needs genuine.

Thanks for confirming my gut feel - Factory Oil it is - so anyone got a tip for least Wallet Gouging Mitsubishi dealership in the Brisbane South to Gold Coast area?

Castrol make an oil specifically for Mitsubishi transmissions. VMX-M, costs around $44 for 4 liters. Though if you can get the genuine stuff for a similar price that would be the go.

Is that a Manual oil? seems like it used even in 4x4 manual boxes?

I used UltraTune to get a full transmission flush. Not sure if they used genuine Mitsubishi fluid, but I do know that I had an occasional somewhat violent shudder that appeared randomly on smooth roads. That completely went away and I havn't had issues with the transmission. Cost was about $230 I believe, I figured fairly reasonable since it would cost nearly that much for the quantity of oil needed, and there's no way to properly flush it without a machine

Yep. I lot of people use it in other makes when they have syncro problems as the Mitsubishi oil has different properties which can improve shifting. I use it in my 4wd as well because it is only GL 4 and does not contain a sulpher additive like GL 5 does which can eat into brass syncro cones.

I should have added my car is an auto so me thinks I'll not use the manual gear oil.

Good choice. Castrol used to do a fluid specificaly for the auto Mitsubishi's, I had to get it once as I was half way through a flush and needed more to finish it on a Sunday when the dealership was closed. It cost more and was only 4L. They don't make it anymore, only have a multi-vehicle oil which I'm skeptical about.