Hi guys,

I want to replace the oil in my ralliart magna, 5 speed auto...what oil is recommended?

The oil was changed on its last rebuild but it has been overfilled so for piece of mind I want to freshen it up :)

Genuine SPIII Mitsubishi Oil.
$44 per 5L bottle from your local dealer

Awesome, how many litres will I need?

Sorry answered the question, 5ltrs, brain fart

No you need more.
I think the whole transmission including torque converter is 8L or so
However if it is old oil and/or you do not know if genuine AT fluid was used you would need to flush it.
To give you an idea the two times my transmission was flushed I needed 15L and 12L.

Everyone will advise you to flush it

What is the process in flushing it? Somehow it doesn't sound as straight forward as dropping the oil and replacing it, like that of engine oil.

No it isn't.
If you just drain the oil and fill it a lot of the old oil will be left in the system (torque converter etc). From what I've read it seems nearly half the oil will remain in the transmission this way.

Flushing it basically means that while it is running and pumping oil around, you feed it new oil while it is "dumping" out the old oil. You stop this procedure once the oil that is being dumped starts coming out clean.

I believe (I may be wrong) that you use the hose that comes out of the transmission and goes towards the radiator for cooling for this.

If you search the site there are quite a few posts describing how to do this in detail.

Alternatively (and if you do not feel confident or are not familiar with working on cars) get a transmission shop to do this for you. The labor should be about $150 give or take a bit, and you'll need about 3 bottles of ATF at $44 each.
In the 2 times I had my transmission flushed I bought 4 bottles and refunded the 1 that went unused.