I think Dagwood's correct, newer 'conventional' oils are much improved.
But the newer oils are really not conventional, the have better basestocks and they have to pass more rigorous tests. Most are, or are close to being, 'semi-synthetic blends' for lack of a better term.
In terms of price [ or value /performance ] the newer oils do undermine the advantage that Mobil 1, Redline, Amsoil, and GC enjoyed.
Unless your engine needs the better synthetic oils, or you drive really hard, or you want a longer drain interval. This is especially true when a manufacturer requires a 'target spec' oil or a factory approved oil rating.
The new conventional oils have lower evaporation, but not as low as the 6% NOACK number for Redline. In a fleet for a large family or a business, ultra low evaporation means fewer problems if drivers don't check dipsticks.
The newer oils do have much wider temperature ranges, but not as good as PAO/ Ester synthetics.
Group II and III might do better with keeping some deposits and additives in solution than Group IV and V...but this is probably not a big advantage with modern unleaded gas engines.
I agree that for many vehicles a new conventional oil could do nearly as well as a true 100% synthetic...if shorter OCI's and ~perhaps~ a winter weight and summer weight, or a normal weight and a severe service weight are used.
Shorter OCI's have advantages, especially if glycol or dirt get into the engine. This is true if one has the time to do more oil changes and the control over the vehicles.
So I think the premium full synthetic oils no longer have a clear advantage over conventional oils. When the gas to liquid basestocks like Group III+ become more common the situation will become even more interesting.
I could imagine Group III+ with a little Ester and/or some PAO or newer 'PAO like' or 'ester like' alternative like EOP or Supersyn or some yet to be released product might offer even better performave and value.
For now, I'll stick with mostly Redline, Mobil 1, Delvac 1, Amsoil, Pennzoil Platinum, and GC for synthetics, and in certain vehicles: Delo 400, Motorcraft, and Havoline for conventional.
And I'll ARX. and keep adding LC20 and FP60, and a little VSOT, or SX-UP