Why do you feel test has the highest chance for sides? Its naturally in your body, men start to feel Bad when they have no or very little test, depression, no sex drive (libido), lack of energy, get fat and weak.
Alright time to explain this again. Deca, tren, anadrol ETC, ETC. when taken will interfear with your HPTA. This is a negative feed back loop within your body. Your body has a natural set point of the amount of test or androgen's that should be present. When it senses excessive amounts of androgen's are present like it would be if you started taking deca, it sends a signal to the hypothalamus to tell the testes to stop making test, this is known as, shut down. Its kind of like the thermostat in your house as a rudimentary example.
Well the bad part of no testosterone in your body is that you will not have normal sexual function or desire. Thats why all cycles need to include test, the synthetic test replaces the naturally occurring test that is no longer present because of the HPTA shut down. Also increased test levels will lead to strength and size gains.
I have to ask you this, if you only wanna put on 5-10 lbs why don't you just stay natural. Seems to me somebody could put on that kind of size the natural way with some hard work and some time?
My explanation was short and just very basic I urge you to do some web searches and reading on these issues for yourself to help you decide better.