I've had an IZH 46 and a 46M over the years. Both arrived "bone dry", and my experience is that the internal parts ought to be left that way. A little bit of oil on the trigger contact points might be helpful. Light machine oil is good, or "pell-gun" oil if you have any. I never cleaned the barrels because they never got dirty. These are less expensive competition grade guns, and the velocity in not enough to qualify it as a firearm. I don't think you will find any build-up of lead (or anything else) in the barrel. Cometition grade pellets are the best for accuracy. The gun was made to be competitive at 10 metres. In ISSF matches all you need to do is to put one hole in each target. Nice crisp holes punched in a target are not what you will get with a 46 or 46M. Slower pellets tend to tear paper, but as long as you can see where the pellet entered clearly, that is all which is necessary, and it does that very well. Flat-topped pellets are the best: JSB pellets work, so do RWS, or H&N. They cost more, but they show you how you are shooting. The best I ever shot with my IZH 46, was a 578/600 (at a local competition). If you don't like the grip, don't modify the grip that comes with the gun, find one which will fit you properly. That is time and money well-spent.
Steve