Make sure the 2 drain plugs are fitted with copper washers and are tight. Remove the gearbox breather [5/8" AF] to fill the gearbox with oil. The reversing light switch is not the place for this unless you like a mess, as the 5th/reverse gear selector shaft is right below and in the way of your oil feed pipe. Remove the dipstick from the crown wheel casing to allow displaced air out as the oil pours in. I made a pressure oil filler using a discarded air suspension unit from a 44ton trailer, into which I pour the chosen grade and quantity of oil, pressurise it to 30psi, insert the feed pipe into the breather hole, and open the tap, blowing in the oil in seconds. There is some discussion regarding oil grade and quantity for these gearboxes. I pumped in 2 litres of SAE140EP gear oil. The level is somewhat above the max mark on the dipstick, which does not worry me, but you don't want it to run out of the first motion shaft onto the clutch, so BEWARE!
It's worth reading the JKTowers article about the Talbot Express gearchange linkage, which uses a very similar gearbox. I agree with his comments regarding the quantity of oil needed, but disagree about the grade of oil he suggests. A gearbox in a loaded van or coachbuilt caravan in a warm climate, needs a heavy duty gear oil of EP type, not a lightweight oil. The viscosity of heavy oils seems to fall off after a few months use, easing the initially heavy gearchange. I also agree with his suggestion to keep the speed down in 5th gear and to change down early on hills.
As an aside, I disagree about the oil Fiat recommend [Selenia] for the 1929cc turbo diesel engine. In all my vehicle engines diesel or petrol, I use Cepsa or Ertoil SHPD SAE15W40 mineral oil formulated for hardworking turbo diesel truck engines up to 600hp, and I never have engine trouble put down to lubrication issues. Bought in 50lt. barrels it's cheap enough to change often. Synthetic oils have suffered from patchy formulation in the past and are also beyond my pocket, so I keep well clear of them. In a cold wet climate like UK condensation and acidic buildup ruins any engine oil within 5000 miles anyway, be it synthetic or mineral.
I wrote the above paragraphs in my 'How To' regarding the Ducato 10 gearbox so here it is again, copied and pasted. Fiats often come from the factory with insufficient gearbox oil, so be careful, it may never have been changed or even checked before.
