Multipla stiff gear change - 2005 Fiat Multipla 1.9

what is stiff (add a noun here)
blush.....
clutch pedal stiff
shifter lever stiff.
grinds , a bad clutch will make the lever seem stiff or will grind, if forced.
gears balk? same as stiff.
false neutrals. (slips , gear not engaging)

Drain should be at the lowest point of the trans, with fill plug about half way up on one side.

I presume this is a 5 speed manual transmission? Is it actually the gear lever that is stiff to work through the gear changes?
first thing to do is drain and refill the gearbox with new oil.
while you are under there, check that the linkages are not rusted or crudded up. If so, free them up and lubricate them well.

Yes you can. As long as it's a diesel engine and the same model range (Multipla).

you may have snapped a gear band? I've had that problem before.???

Are you sure you installed clutch plate facing right way? They are either stamped or have a tag says "flywheel side". Or might have wrong clutch kit.

I am not familiar with that Fiat but found an inexpensive manual http://global-data.co.uk/products.php?product=Fiat-Multipla-Workshop-Manual&setCurrencyId=2 I am not affiliated with them. I just thought if you needed it. Also FIAT forums may have your info.

try changeing the detent device ontop of the gear box. it is a 24 nut when u open it there will be a ball and spring attached to the other side. it is located ju8st behind the gear selcator on the very top of the gear box

You can try using synthetic gear lube, I have found it reduces shift effort by at least half when it is cold.

In all these cases it appear to be a problem with the synchronizers or a dented pinion. To verify, the oil can be check for metal deposits. Fiats have bronze synchronizers and when placed under heavy shifting load they were out rapidly. if this is the case replacement is the solution and transmission has to be open. The car can be driven but gear will keep releasing. Sometimes the synchronizer level bends causing an incomplete setting off the gear.