Oil goes missing sometimes

I have a problem with my 96 4efe engine and that is the oil goes away sometimes (the oilstick is dry) but when I park my car and let it rest for a few hours all the oil comes back.

I do mostly city driving these days and thats where the oil problem shows up and when I park my car at night there is no oil on the ground the next day, I use Shell Helix HX7 10-40 oil and I have to put about 1L oil in it every 6-8 weeks, I did not/do not have the problem when driving on twisty roads.

What can I do to keep the oil in the engine?

I do plan on spending alot of money on my car in the long run but an engineswap is out of the question (that would make my car illegal to drive on public roads according to the rules and regulations set by the goverment in Norway)

in all honesty i dont know, maybe the oil is thining out too much?
have you got any blue smoke?
are you sure the dipstick is going all the way down?
when you say returns, does it come back to how much it used to be?

miib: The oil might be thining out to much, there is no smoke at all, the dipstick is as far down as it will go and the oil returns to the level it was at when the engine is cold.

I'm not that good at explaining things in english.

That is weird... I honestly can't really think of anything.

Thats why I posted this thread, I hope someone can help me, the last thing I want to do is to modify the oilpan.

That's Odd, first thing I thought it could be is the Oil Pump