I have a 1998 pajero io which smokes when in traffic for 20-30 mins,problem started after servicing with a synthetic oil.

First you need to find out what is causing the smoke. What colour is the smoke?
White Exhaust smoke is usually coolant getting into the cylinders.
Blue Exhaust smoke is usually Oil getting into the cylinders and burning with the fuel.
Black Exhaust smoke is caused by the fuel to air mixture being too rich. This could be your Air Filter so check it is clean.
A common cause of Blue or White smoke is a blown head gasket. This can also show up as oil in the coolant tank and/or a 'mayonnaise' like buildup inside the Oil filler hole.

if you have been idling for a while ,like at lights , and the smoke appears when you accelerate off, that indicates that the valve stem seals have failed allowing oil to be sucked down the valve stems into the combustion chamber
If the smoke appears under load as in going up hill or the like , then that indicates excess big end bearing clearance which is overloading the oil ring and loosing control that is bypassing sump oil into the combustion chamber

White smoke is usually coolant entering the combustion chamber indicating a bad manifold gasket, head gasket, or cracked head. Blue smoke is oil entering combustion chamber or excessive fuel combined with trace amounts of oil. Indicates worn rings or valve seals.

engine burning fuel.
not good sign

a few things could be the cause , one is glow plugs , two is injectors are dirty which usually gives black smoke out exhaust , three is injection pump leaking , and four dirty fuel filter . these are things I would check .

Most likely- oil getting past the rings. Regular oil changes lately? Have you tried synthetic blend oil? Or full synthetic? There are also valves and gismos and thingy's on the Nissan and Infiniti that go bad and could cause same thing. Even your air filter, is it clogged?? Air flow meter and switch ( sticky or bad switch) also on those get varnish deposits and your air "flap" for your induction may not be opening properly, a good intake cleaner could help, and the cheapest to try !!!
Do you sit in a lot of traffic? Not a lot of full running? Low quality or wrong octane gas? Back to the air flow, it could be "choking" like on the old carb jobs. Dirty and the choke does not open fully or gets stuck, gave the impression you were burning oil, but it was just burning too much fuel at startup and it is a similar odor. Same effect with air flow meter and flap. Worked as a Nissan mechanic for almost 20 years in Wash DC and this was common because of traffic and idle times. Oh, and cheap and wrong gas, very important for the long run.

Hi can you tell me make and model. Does the car blow blue smoke all the time and other than smoke any other symptons? poor starting , power loss etc.
I had a similar problem on a golf gti after servicing drove very well but smoked on start up, and found to be the high grade fully synthetic oil recommended by the factors was simply too thin, and thus giving me by pass on worn valve seals and pistons on older engine sometimes a thicker grade oil is better.

EGR valve playing up ,just cap it off with nice blanking gasket made out of a coca cola can job done and sorted

sound to me like it may need the head gasket replacing, check for the following, oil in the coolant, coolant in the oil, leaks of oil from the head, not the rocker cover, check your spark plugs for oil, if it is not the head it could be oil contamination in the cylinders above the pistons. hope this helps

Some people change oil at 3000 miles,most people don't really care or keep track
I would say 3 times a year,if you drive less than 10,000 per year
4 times a year ,up to 20,000 miles per year driven
Use at least semi or full synthetic oil