Thanks to everyone for the responses. I'm pretty sure that I won't lose on this truck.
I've gone through the receipts that came with the truck, from 116,000 Km to present @ 215,000 Km. The rear diff was rebuilt @ 180,000 Km 3.73 gears, auburn limited slip. The spark plugs were changed and the fuel system gone thru @ 185,000 Km. Actuators [4x4?] and front bearings @ 202,000 Km. It seems there was regular oil changes ~ 5000 Km. The last receipt is for diagnosis; code p022 - cam position timing over retarded bank 2, may need t/chain, belt, variable cam timing actuators, and chain guide. And garage replaced oil sending unit and bank 2 variable cam timing solenoid at that time.
I pulled the codes [unsure of procedure but wth, I gots to lern sometime lol] and got these the first time;
p0134 o2 sensor circuit no activity detected bank 1 sensor 1..... is this indicating the first o2 isn't working or maybe the wires are dislodged?
p0171 and p0174 system too lean banks 1 and 2.... maybe has something to do with 1st code?
After fiddling with reader I pulled another code;
p0022 a camshaft position timing over-retarded bank 2..... the same as the garage pulled.
I don't have enough experience for diagnosing the problem, I'm a 50 year old playing with cars and maybe learning something along the way. I will buy the manuals needed and if someone is kind enough to give directions, I can and will fix the problem, I don't give up easily. I once pulled the ac from a '91 ranger without letting any gas escape because I was told it couldn't be done, tho it did take me quite some time....and it never got put back in.
After putting in the a fore mentioned 7.5w/35 oil I went for a 100 Km drive and the truck ran great until coming to a stop in gear. Oil pressure was @ 55-60 psi at cruising speed of 110 Km/h, 1800 rpm, not fluctuating except in relation to rpm. At a stop oil was @ 20-23 psi, 500-650 rpm. Rough running was experienced when oil dipped below 23 psi and idle dropped rpm in relation. Feather the gas and idle smoothed out. Turned around and drove back no problems, gas mileage indicator showed an average of 15.2 L/100Km over the whole trip. No codes came up after the trip. There are no problems experienced until an extended driving time.
I'm inclined to do as berry1234 suggested; de-sludge the engine for 1000 Kms and next oil change I'll use 15w/40 as I happen to have some synthetic of that grade I picked up for cheap. When I ran 5w/20 the motor ran like crap warmed up at an idle, I didn't have a oil pressure gauge installed at the time.
OTOH it may be best to tear into the front of the motor and fix the possible blown chain tensioner 'gasket' as suggested by Sasquatch_Ryda. Can anyone give me an estimate of time and parts needed for this?