the gearbox oil will not need doing every six months unless you are doing starship mileage like susan.
However the service intervals on them are now 25000 miles with "long life oil" , i would personally change the oil every 10,000 as oil in poor condition will wreck the turbo eventually .
Water gets into the gearbox via the gear selector shaft on those early model vans. Its the water that does the damage to syncro cones and bearings. So Susan's suggestion of replacing gearbox oil regularly to a good one. Changing the oil seal on the selector shaft is also a worthwhile cost. Early Ducatos/Relays and Boxers suffered from the same problem and the seal was easy to replace.
(The strange thing is that not all vans suffered with this problem although all had the same box.
The solution for the Fiat manufactured vans was to reroute the drain pipes from the heater air intake as they ended above the gearbox. The other thing was the the gearbox was strengthened/reinforced by a pattern of ridges that looked like small boxes and was part of the gearbox housing casting. These 'boxes' collected water, mainly from the drain pipes. So each time it rained these boxes were supplied with fresh water. The gear selector shaft was located in one of these 'boxes'. Even someone with half a brain could guess the result. The short term solution was to reduce the height of these boxes with a grinder and then fill the remainder of the box that had the selector shaft with a resin so that water could no longer pond there. As gearboxes are usually made by a specialist company, it wouldn't surprise me to hear that Reanult used the same gearbox in their van as did Fiat.)
As far as servicing intervals are concerned, look for the small print. Somewhere will be something that advises that the vehicle should have an interim service every so many miles or annually, which ever applies.
Safety is one of the big issues with regard to long mileage service intervals. If a mechanic advises that the front disc pads have 50% wear, what does he do? Does he change the pads as they won't last to the next service (adding cost) or does he ignore them, hoping the brake warning light is operating on the disc pad with the most wear?