Volvo Repair: fuel consuming, bosch service center, fuel tank cap

Question
yes dear phill I am the origional owner of this car since 1992 and it was running fine , you are taking right and I asked about the o2 sensor they said to me this model is without o2 sensor and its called B234G and I have to use unleaded full only ( its written on the fuel tank cap ) and what make me believe that there is no any electric connector around the axhaust not connected anywhere  , and the axhaust system on it is the origional one since 1992 when I bought the car and its a volvo mofflers and condencers , thats what make me believe that the viscosity of the oil may effect the lifters of the intake valves , I am in KSA ( kingdom of saudi arabia ) , do you think its origional like that or maybe the volvo agency take out the catalytic converter and the o2 sensor at the begining ? and I doubt in that cause the OBD diagonistics system will figure that out and give me a code for missing o2 sensor signal and that is not happen with me , and if its origional like that do the viscosity of the oil effect the opertaion of the intake valves lifters which cause that ? and what is the proper fuel pressure in the injectors rail cause I doubt in this ?
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Followup To
Question -
hi dear phil ,
  first I like to thank you alot for your help , I have a volvo 940GLT 1992" with B234F engine ( 16-valve ) and with jetronic 2,4 fuel system with air mass meter (016) but without catalytic conv. or O2 sensor , my car consumes too much fuel (about 100km/20liter intown driving) and the CO axhaust outlet is about 6% (too high) and the engine have poor idle (unstable idle speed between 650 and 850 rpm and some vibrations) I flush the enjectors at BOSCH service center and clean the inlet valves and champers from carbon use special chemcials and checked all the engine sensors (AMM , TPS , ECT ,atc) and its all work properly and I make a full service and still like that , no vacum leak , no hesitation at drive but the power is a little bit low , what do you suggest me to do ? I am gonna be crazy , and I doubt there is a decreasing in fuel pressure so may this cause all of that ? and how much the pressure should be ? and if I use 20w50 oil do that cause this symptoms ? do I have to use 10w40 ?
Answer -
You have a weird one here.
This car is supposed to have a cat converter and o2 sensor, how else can it monitor the exhaust  and how can it go into closed loop. These cars run near zero CO%, so 6% is way, way too high.

Are you original owner or has this car been tampered with?  What area is this car from?  

Answer
Well, that answers part of the question. I'm from the US and have no experience with foreign models. Not even sure what style fuel injection system you are running, so our specifications here may be different than yours. You'll have to refer to manual.
As far as your oil is concerned, that should not be the problem. My guess is a bad fuel pressure regulator (if you even have one ) Pull the vacuum hose off the regulator and see if it's wet with gas, this means it's bad.
Another thing to check is MAP sensor to make sure the vacuum hose is still attached.

Let me know what happens and I'll try to research about your car.