low oil pressure(zero when hot)300e?+3 on Highway though

[petaling]

Problem 5 months.Not new.No castrophe yet.Does this fact alone says that there is oil pressure but not registering?????

Low pressure getting lower and lower to zero now.

Morning start-up after 1-2 sec Max pressure.
As engine heats up,gradually idles at zero pressure.No increase in oil comsumption,just 1 litre per 3-6 months as before.NO massive oil leak.

I've read that it would be the sending unit,oil pressure relief valve,main bearing ,oil pump.(the last two meaning big money)

My moonlighting MB mechanic told me as long as there is pressure when you accelerate and the needle goes up,dont worry.Just drive it.Dont spend money.

Any advice as what the cause maybe or anything that small amount of money to fix this.
Anything approaching 1000$ would mean a trade-in for me.Should i change the sending unit as a diagnostic test as I dont have tools to do pressure test etc.
Thanks in advance.