Many of the additives used in modern dyno oils over the past few years are a lot better than used in the past; thus, the modern "Starburst" symbol dyno oil lasts longer and is more stable than oil used even five years ago. Unless the engine is of a race design, or has limited airflow to help cool it, dyno oil is all that is required. GM (and all other manufacturers) recommend the oils that they test the engines with during development.
Synthetic will not harm the engine, but in many cases, it is not necessary. Extreme hot conditions and extreme cold conditions will see some benefit, but then again, the GM oil life system, which monitors temps at startup and during engine run, will take this into consideration. Therefore, those who live in AZ will see their oil life percentage decrease faster than those living in MI during the summer. Same holds true to those up in Alaska. Those living in moderate temp areas, with four seasons, will also notice that during the summer their oil life percentage will not decrease as fast as during a cold winter.
You will also notice a difference with your driving habits. Change the oil and go cross country and your oil life percentage will drop slowly. Change your oil and make a hundred one-mile trips to Kroger a mile away, and your oil life percentage will decrease faster.
In addition, there are two primary type of synthetics; Group IV synthetics are made with PAOs, group III are highly refined dyno oil via a process called Hydro-cracking, that is labeled synthetic oil, and is legal to do so. (Mobil sued to have the name synthetic removed from Group III oils back in the late 90's and lost.) Hydro-cracking is less expensive than using PAOs. To be honest, the inroads in the development of Group III oil have made them almost as good as Group IV. There was supposed to be a Group III+ category, but not sure if it has been released.
Last tests I saw showed that only two oils were group IV:
Mobil 1
Amsoil ASL
All other major oils, advertised as synthetic were Group III:
Castrol
Shell
Pennzoil
Valvoline
Havoline
Quaker
It costs a lot less to manufacture Group III synthetics, but as we all know, they charge about the same as Group IV.
Now, to add confusion, there is a Group V synthetic oil based on Esters, and is used by Redline. The only true benefit of Group V is that it's high heat capabilities are superior to Group III and Group IV. A normal car/truck engine will not benefit by the use of Esters, but it certainly won't hurt. Air cooled and Jet engines benefit greatly with a Group V oil.
Another type of oil that is the "blend." When you mix a synthetic with a dyno oil, the oil takes on the characteristics of the lowest group type used. Therefore, a blend oil is basically worthless. If you are considering a blend, you might as well save your money and go with a dyno or pay more and go with a synthetic.
Most important factor, is always use the recommended spec, or a spec that exceeds the spec for your engine. This is important on systems using oil life monitoring systems. The system algorithms are developed knowing the rate of decline of the oil approved with that spec. Using an oil without that spec would make the system inaccurate. So, the oil life indicator might indicate 50%, but the oil is really degraded to around 30%. Using an oil with a greater spec (synthetic), would also make the oil life monitoring system inaccurate, but won't harm the engine. With an oil that exceeds the spec, the oil life system may show a percentage of 50%, but the oil life is actually at around 70%.
If you are really curious, run dyno oil for 6K miles and send it to a testing lab like Blackstones (Indiana), and for twenty bucks they will test it and send you a full report. Look at the TBN number, the higher the number, the better the oil has remained. Then do a synthetic oil change, run 6K and do the same thing. Unless you live in an area with severe weather, you will notice little differences between the TBN numbers.
One of the most important things about oil changes but are greatly overlooked are the filters. Changing your oil and installing a cheap filter is not a good idea. The testing I saw years ago (by an independent firm), had three filters at the top of the list:
AC Delco Ultraguard (NO LONGER IN PRODUCTION) filtered down to 8 microns
Mobil 1, filtered down to approx. 10 microns
K&N, filtered down to approx. 12 microns
All filters not only filtered down to the lowest number of microns, but also allowed oil to flow at a required rate. All three of these filters used synthetic filtering media.
The latest test results I reviewed, showed the following:
AC Delco Duraguard (Champion)
Mobil 1 (Champion)
K&N (Champion, I believe)
Fram
Tough Guard not the stock Fram (Allied Signal)
Motorcraft (Purolator)
WIX (Purolator)
Filters to avoid:
Fram
Extra Guard
Fram
Double Guard
Pennzoil (By Allied Signal)
Quaker State (Allied Signal)
AMSOIL not tested (Baldwin)
Most filters are manufactured by Champion, Baldwin, Allied Signal (Fram) or Purolator; however, they are all manufactured to different specs issued by the purchasing company. (Allied Signal actually makes Fram filters for use by Fram and other suppliers who purchase the filters from Fram.) The important thing to note in a filer is the filtering media, how much surface is used to filter the oil, a strong metal case to withstand the high oil pressures, and a bypass valve as well as an anti-drainback valve (if required).
The right paper media does a good job filtering while allowing for sustained oil flow, synthetic media allows for smaller passages to filter smaller particles while still allowing required oil flow.
The bypass valve allows the oil to bypass the media if it should become plugged or during times of extreme cold startups. Better to have some dirty oil circulating in the engine, than no oil.
The anti-drainback valve is not required on all vehicles. For example, with the oil filter mounted vertically, the filter will remain filled at all times. However, when the filter is mounted horizontally or on another angle, some oil will drain back into the engine when oil pressure is zero, so it takes a few seconds to fill it when startup. The anti-drainback valve prevents oil from draining back into the engine. Most filters in this day and age, have both the bypass and anti-drainback valves.
Now that I have said this (again), I will admit to running Mobil 1 with Mobil Oil filters; however, I do not do it for the synthetic oil, I do it because I have gallons of the stuff. A few years ago, before leaving TN, PEP boys had a 1/2 off sale on Mobil 1, and they had no limits. Therefore, I filled my Jeep with cases of oil, and last year, I filled it again when I moved to MI (Movers would not move flammable liquids).
I only use Mobil filters since Ultraguard was discontinued. I would have no problem using K&N or WIX, but the auto zone near where I work does not carry &N or WIX, so I go with Mobil 1.
Enough said. Like GLBLWARMR stated, you wlll get get alot of different answers on this question.