The only way you can check the level is by removing the speedo from the gearbox. It is located right at the very back and on top of the gearbox - pretty much the last thing on the engine.
It's not that difficult to do and shouldn't take too long.
First you will need to remove the entire air filter housing (only 2 bolts and a clip). Once this is removed you should have no difficulty seeing it. What you will then need to do is unscrew the speedo cable, you may be able to do this by hand. Once you have pulled the cable clear you will see the top of the servo. To remove the servo you will need to remove the 10mm retaining clip. Once this is out take a pair of pliars and pull out the servo. If it is in tight just give it a little tap with a hammer and it will come out.
Shine a torch into the hole and the oil should come up to the lip inside.
However, from what you have said it sounds like your gearbox is on the way out and will need a new one. As the gearbox uses the same gear for 1st and reverse you will have problems with these when using or attempting to use them.
Don't use ebay or any other place when you buy a gearbox, look out for Nissan Micra enthusiasts websites as they often have parts for sale as bargain prices.
I know this as I had exactly the same problem and only a new gearbox fixed it (which I picked up for £30 from a Micra enthusiast!)
Clive
August 2010
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