Gases get into the oil sump

While it isn't a good idea to block the tailpipe it won't cause much of a problem at idle speed as long as it isn't blocked for a long time. The exhaust gases will back up in the engine and start being released out of the intake valves into the intake manifold because the pressure in the exhaust is higher than atmospheric. Some will also be forced through the PCV system but not much more than normal. What you have basically done is the same as an EGR system does. The difference is that blocking the exhaust also stops the secondary purpose of the exhaust system which is to remove heat from the engine. This is why a failed muffler or catalytic converter can cause an engine to overheat, the extra heat goes into the cooling system.
So you didn't cause any damage but it isn't the best idea. A quick DIY test for an exhaust leak is to use a pressurized air source connected to the tailpipe with a cold engine, like an air compressor or even a shop-vac set up to blow air. Then listen and feel for air being blown out of the leaks. To make it more effective you can block off the air intake on the engine to hold in the air that will go past any open valves.
If you have access to a smoke machine, even the home made ones, those are even better as you can see the leaks.

Hi Steve.
Thank you very much for your great explanation and advise.
I was pretty sure that i need to change the engine oil,thinking that the burn-gasses got into the oil sump and contaminated it,but i understand that it didn't.(as you said:" not much more than normal").
When you said:"it won't cause much of a problem at idle speed as long as it isn't blocked for a long time." What is considered long time? since i thought (after i realized what have i done) that five minutes is too much.

At an idle you could probably let it run a couple hours before it started getting overly hot. It's when you start working the engine and it starts producing a lot of heat that it can really cause problems. I have seen engines being run "normally" that have low power and run hot from a plugged converter, you open the hood and the exhaust system is glowing red hot and the heat is enough that you could easily cook on the engine block. That is when you have caused some damage.
If you want to feel better you can change the oil, that is never a bad idea anyway.
But it really shouldn't be needed.

Thanks Steve, I feel now much more relief after you said couple of hours.
I thought again about the way of the exhaust gasses,as you explained,that they go back to the intake manifold.From the intake manifold does part of them go out through the carburetor outside and part of them (which got from one piston intake valve),gets back along the intake manifold to the other piston through the intake valve?
If so, why didn't (in my case)my engine,at least,stuttered?after all the mixture in the combustion chamber contains exhaust gasses.Oppose to the normal situation that the engine should be stalled or at least stuttered,my engine was running nice and smoothly with blocked tail pipe.

At an idle the amount of exhaust generated mixes in the intake with fresh air and creates a mix that burns well enough that the engine runs. That is because the intake is under atmospheric pressure at all times. At idle the exhaust generated is only above that pressure for short pulses. The remainder of the time the pressure of the air tries to purge the intake so you have a constant cycle of air, fuel and exhaust. It isn't a real good mix for power but at idle speed the engine isn't going to struggle much because the intake design is capable of drawing in far more air than the engine can actually use. Now if you were to block off the exhaust and then slowly block the air intake, you could get to the point where the engine would start to struggle and stall as the mix in the intake leaned out enough that the engine cannot run on it.

Your explanation cleared all my doubts.And i understand from it,that the engine doesn't have to stall or stuttered when the tail pipe is closed on idle even if there isn't any leakage in the exhaust system.
Thank you very much,Steve,for your help it was very helpful.
Now,i just need to find a vacuum cleaner that blow air.
Not an easy task. :)

Most shop vacuums will. Or even a hair drier or similar. Another thing is to look for fresh rust or deposits from the exhaust as well as noise. Or if you can dig up a few pieces like a BBQ tank regulator an empty clean paint can a length of nichrome wire and some tiki torch wicks you can build a smoke machine for testing.
Go look at Brian B on YouTube. I built one to test against my redline unit. The paint can doesn't have the fancy gauges but it will do most of what the "pro" machines will do, at a fraction of the cost.

I wasn't thinking about that, but the hair drier idea seems to be the most accessible for me right now and if i won't find the leakage, as the last resort i guess i will check the option of trying the smoke machine method (since it seems to be the most complex option).
Thank, again,for your great help.I hope i will find the source of the elusive leak. :)

Good luck. Thanks for using 2CarPros. Please return anytime with your automotive questions. We are here to help.