yep... you should run std oil in a rebuilt engine for 5000 miles or so... then you can use synthetic.
Also some pretty smart guys say carbed engines contaminate the oil too quick to get the extended change intervals so std oil is most likely the best choice..
OK, I really hate to bring this thread "back from the dead" but this statement has really made me lose sleep at night.
Exactly how do fuel vapors from carbed engines contaminate the oil in the cranckcase and fuel injected engines donot
The FI engines spary/inject the fuel, have closed loop sensors, computers that read the engine and correct many times per second. The atomization of the fuel/air is much better and its a much advanced combustion. A carb is set/tune for the best of conditions and is set. The FI can take temp, pres, O2 calc for air volume and is just a better combustion. That is why many FI engines get 25 to nearly 40mpg now and a carb will never touch that. Carb engines usually have to set on the rich side or could be toooo lean for some situations, lugging, trailer towing, off road, even min cooling could be considered.
The extra fuel is still in the air/fuel mixture and there is always some blow by. The blow by contaminates the oil on the back side of the pistons/cylinder walls. The hot gases from combustion will also condensate on the cooler engine surfaces internal and then be washed with oil splash etc. That is why we have the PCV system also... the blow by get to the internal part of engine, warm air rises up past the lifter/oil drain area and up to the valve cover. That is where the PCV is, the gases rise up and sucked back into the carb to be reburned. Keeps the engine cleaner and the enviroment. The air cleaner cleaned fresh air usually goes in the back the valve cover to help the engine gases move to front and extract.
The idea with synthetics is the oil does not break down with pres and temps and last longer at the molecular level. The molecule stands between the metal surface and tries to survive and does so much better than dyno oil. But with tooooo much gas the lubrication quality of the oil (both) go down. The oil will start looking black and in very rich run conditions it will smell like oil. If you are BLACK OIL and STINK LIKE GAS OIL change your oil and filter ASAP.
As an example, when I first go my weber carb it ran pretty well but was rich. After 1000 miles my oil was pretty black. When jetted correctly, tweeked my ignition and installed a CDI MultiSpark Ignition, the oil is now clear after 1000 miles or little less. Keeping in mind its a new rebuilt long block and does not have 100k like many of us do.
I learned this caution and thought process from JeepHammer and he is one of the sharpest guys here on JeepForum. You can use synthetic in a carbed engine, our old jeep engines, but you will not get the 6k or 7k change intervals that you could get from a modern FI engine. Also learned that the oil gets acids and moisture contamination with use and these cause corrosion, surface pits, and not good for engines.
JeepHammer said it, I agree, many times see printed change your oil 3000 miles or 3months. When you store a engine for winter storage or a small engine for summer storage store it with a clean filter and oil. If you start to look at your oil at 500 mile intervals you will see the old CJ starts to look older fairly quick, your modern FI may go 1000 miles and still be clear.
So to complete the thought it was the extra cost of synthetic and not being able to get the longer change interval on a carbed engine. Its a extra cost of synthetic vs how long can it stay in the engine.
So I hope that helps and you can sleep better.
I would also like to know what was the "concern" part.
What is the best oil to use in a 304 V8?
Shell Rotell T would be the best choice for our old CJ's, valvoline #2 and this was put on post #2 of this thread, then confirmed a few times. If you want more, Lots of posts on this is you search "shell Rotella T" , Flat Tappet Engine Oil, or ZDPH Oil Additive on this search or google would also give you lots of results.