1996 Chevy Blazer Knocking

is that 234,000? You have been blessed! Most cars with that much on the clock have more trouble than this, just continue with your regular oil changes, once a knock develops, as long as the oil light isnt flashing or on steady, it will be very expensive, and in my opinion not worth the cost to rebuild the engine!

5w30 weight oil is the oil you should run in the winter. 10w30 is the oil you should run in the summer.
I always suggest synthetic oil, here is why. Oil must do three things inside of your engine. First it must create a wedge between moving metal parts. This is how it lubricates. Second it must flow easily to create pressure. Many people don't realize this but oil pressure is not created by your oil pump, it is created by the resistance to flow inside of your engine. The oil passages, bearing clearences, thickness of oil are just a few examples of resistance. Third , this is the most important function oil has, oil must have the ability to carry dirt, worn metal, and any contanimation etc.. to the filter; where it is caught. This is why you should change your oil every 3000 miles. Dirty oil is a good thing because it means that the oil is doing its' job. Synthetic oil far surpasses regular oil in all of the three reasons. I use Moble 1 full synthetic.
The knocking in your engine may be a few things. The cold weather is undoughtingly a factor. However the reason has very little to do with fact that oil is thicker when cold. It's mostly to do with the fact that cold weather makes all the clearences inside of your engine contract. The clearences inside your engine are thousandths of an inch or micro meters. As an engine wears (and they all do no matter how often you change the oil) the clearences get larger. When it is cold, the metal is allowed to contract more, thus causing the oil to spend a little more time( if you will ) creating its' wedge which lubricates moving parts.
Your rocker arms may need adjusting. Your valve springs may be weak. Your lifters may be worn.
All internal combustion engines eventually wear out. It's just a simple fact. If the knocking goes away after it is warm, then you should try synthetic oil designed for high mileage, sorry, high kilometer engines. If it knocks when the engine is warm then that's a different story.. keep me posted