Re: 156 gearbox oil
I changed the gearbox oil on my 1.8 TS last year.
To check for the level, remove the filler hex - the oil is ment to be at the base of the threads.
How to change the oil ......
The official plug for the drain is a nightmare, and you will never get a hex in that space to remove it. (see manual for details of all hex locations)
I was ready to give up when i was told the hex plug for the diff is from the same oil reserve.
This hex is really easy to get to and i did the oil change in about 30mins.
When you refill you will need a long thin funnel hose. Halfords sell some that i used for about £3 and worked lovely.
Do a forum seach for my topic on this around nov/dec last year, and you will find all the details and some pics.