my girlfriend owns a v6 tf auto magna. My parent old tj and brothers th with the non serviceable filter took 5 litres on a dump and refill. I was told it was this too by a tranny place. I changed the oil in my girlfriends car it had the filter i changed that 2 and put 5 litres off mitsy fluid in it. Gears felt perfect for 2 thousand km. Just recently 3rd to 4th feels funny sometimes youll be driving slowely and it will either go into 4th fast or sometimes slightly slip into gear or jerk could describe it too. Anyone know wat this could be? All the other gears are perfect. thanks kurt.

Tranny "drain-and-top-up" is not the recommended procedure for changing ATF in a Magna. Dropping only the oil pan means that oil in all the passages of the transmission, in the valve body and most significantly a large quantity of fluid in the torque convertor does not get renewed.
You have diluted old trans fluid with a quantity of fresh fluid, that is all. Magna's seem to be sensitive to the condition of ATF and very very particular about which specification of ATF is used (should meet Mitsi SP3 specs). Common Dexron ATF is said to kill Magna boxes.

As to the gearshift issue, what is the level and condition of the ATF in the box now? If level not quite correct, or (as I expect) the fluid still looks a bit dark and smells a bit burnt - then change of transmission fluid the correct way is best bet.

Lots of threads in here about correct procedure for ATF changes. Buy 3 bottles (15 litres total) of Magna ATF (just as cheap as Mitsi spares as anywhere else), and expect to use about 12 litres.

Fluid is drained by breaking a join in the oil line to the transmission cooler at base or water radiator, and a drain/flush then top-up procedure is repeated until the fluid begins to run clear into the drain bucket. Also a good idea to cycle gear selector thru all positions during the "flush" phases, which makes sure that every passage in teh valve body has old oil flushed out by new. Critical that box is not run dry during the drain/flush phases.

Search should turn up better posts than mine about how to do it, or Workshop Manual thread will give the Dealer Workshop Manual for TH onwards - you can follow same procedure for the TF (and change the spin-on filter which later cars lack).