The conventional wisdom is that you need to remove the battery and battery box in order to change the oil in your manual gearbox. Removing the battery/box gives you access to the fill plug on top of the gearbox casing. Well if you have the 2.0T with F40 gearbox (all 2.0T's with a manual gearbox have the F40), then battery/box removal simply isn't necessary, and thus you save a lot of time and hassle. If you have the F40 but have an engine other than the 2.0T, then this method may still work, dependent upon whether any engine gubbins are in the way. Read the instructions below then check your engine - you'll soon see if it's feasible or not.
1) Look down the left hand side of the outside of your battery box and you'll see the top of the gearbox casing. On that is the oil fill bolt/plug. It's the rusty looking round thing circled in white in the picture below. Haynes says you need to remove the battery box to access this fill plug. Well if you can see it already, why would you remove more items?
2) Before I started this task, I bought a new drain plug and a new fill plug from Vauxhall. In fact, they are both the same part. TIS says that if you ever remove the original non-washer drain plug that was fitted at the factory, you need to replace it with a new one that has a washer, and as it only costs about £2, I was happy to do so. The old fill plug on mine needed a T50 Torx bit to undo it, whereas the new one (and the old drain plug) need a T45, so have both sizes to hand.
3) To access the fill plug better, I removed the plastic clip that keeps two rubber coolant hoses held in place, so the hoses could be moved slightly. I also unclipped the electrical connector that sits next to the fill plug (see first picture), just to improve access. Just pull out the clip on the bottom and pull the plug away. Now thread a T50 Torx bit and ratchet past the coolant hose that's next to the battery box and undo the fill plug. It shouldn't be on too tight. Loosen the fill plug but perhaps leave it in place for the moment, to stop debris falling into the hole.
4) Now jack up the car from the front N/S, and support on an axle stand. Then from the front of the car, reach backwards under the car and find the gearbox oil drain plug. It's already removed in the picture below, but its location is circled in red. Mine needed a T45 bit. Position a bowl or whatever you're using to catch the old oil, under the plug. On my car the plug was on stupidly tight, and in fact bent the T45 bit. But I had another to hand, and this one survived the job. Remove the plug entirely and let the oil drain into the bowl, as in the picture below.
5) When the oil has finished draining out, wipe the threads clean and screw in the new drain plug - this one will have a metal washer on it, unlike the original. Do it up tight, but not stupidly so. Carefully slide out the bowl of oil. The old oil from mine looked almost like cherryade. There are several forums about that say Vauxhall have been putting power steering fluid in the gearboxes, rather than gearbox oil...it might be an urban myth, but what came out of mine doesn't disprove it.
6) Time to put the new oil in. What oil to use? I was going to buy transmission oil from the Vauxhall dealer, but decided to use Castrol Manual Transmission Fully Synthetic oil instead, as it's probably at least as good as the Vauxhall stuff if not considerably better. How much to put in? Whether your F40 had 3.2 litres in it, or 2.4 litres, put 2.4 back in - it gives a smoother gearchange, and the reduction is approved by Vauxhall (TIS field remedy 2263 IIRC).
7) Here's the "clever" bit that really shows that removing the battery box is pointless. To put the new oil in, because the fill plug is low down and access is limited, I used a narrow copper pipe, put a funnel on the top end and inserted the bottom end about half a cm into the fill hole - see the two pictures below:
8) Simply pour the new oil into the funnel, being careful not to go too fast so as to avoid it spilling over the edge of the funnel. Once you've put 2.4 litres in, remove the copper pipe and plastic funnel, and put in the fill plug (the new one you bought from Vauxhall). T45 bit again, and not stupidly tight.