I start it and it goes right at the top.
I tought it was a faulty instrument cluster... You know, rusty engine harness and all.

Thanks for making me doubt!

Newb question : is it bad? How should the oil pressure be?

 

Cold engine oil pressure at 5 (top of scale) is normal / good. Fully warmed up engine oil pressure at idle is typically 2-3 going up to 5 with engine speed.

Sounds like you are good to go.

 

... it actually never go down. It starts at the top, and stay there.

 

Does it go up when you turn on the key or does it show low / bottom of the scale when you turn on the key but go to 5 when the engine starts?

 

It stays at the bottom untill the engine start. Then in matter of a 2-4 seconds it get to the top.

 

That sounds a little strange but if you have a good oil pump and higher viscosity oil it could stay at 5 regardless. Are there any signs that the oil pressure is not actually that high?

Others should chime in but I think you are good to go.

 

Actually, I dunno what oil there is in it, and I have no clue if the oil pressure is 5 or not.
The engine turn smooth and doesn't appear to heat.
As soon as it's running again, I will go flush it for Mobil 1 15-50 (Reading thread around, people seam to agree it's the oil to use). From what I know, there could be a mix of different oil in there.

 

If it doesnt drop some when hot right through (ie at least 20-30 mins running), sounds like a wiring issue. I have heard elsewhere of cases where crossed/shorted wires at the sensor (near oil filter) can cause gauge to stay on 5 all the time.
jp 83 Euro S AT 51k