here is my 2 cents
first off i don't feel that pressure is the right measure for judging your oiling system. what really counts is the volume. understanding the dynamics of pressure vs flow volume is key. you can have great pressure but there is not enough flow/volume to keep all the items lubed properly. pressure is just the indicator that you do have oil in the pan. even if you were at a stop, in gear, a/c on etc: and your gauge showed only 1 bar you'd have near 15psi so you know you have oil flowing. then when you step on the gas and the pressure goes up to 3 bar now you're at 45psi. where is the critical time in this situation? the engine needs oil when the engine is up in the rpms and when you first start in the morning.
as for changing oil weights the only increase or decrease will be in the first few moments of warming up the oil. once the oil is warm you're right back where you started. and talking about multi-viscosity oil the fact is the structure or the viscosity does not change. what that means is the oil is rated ok in this range meaning it will lubricate properly when the viscosity needed is 20w thru 50w.
so pick a number 20w50 for summer and say 5w40 for winter or whatever you think is right.
as for your gauge it's most likely to be a mechanical adjustment in the needle (you have mentioned you took it apart) or something somewhere is grounding out.
good luck with the problem
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