I don't think there is any question as to whether Mobil and Amsoil synthetics are better than regular motor oil. All the specs show that synthetics are better. The key question is, as you point out, "Are they worth it?" I think of it as a treat for the car and nothing more. I do the fluid changes myself and only buy Mobil 1 on sale plus I joined the Mobil rebate club to help narrow the cost difference.
glenmore
1990 LS400
1991 300CE
2000 C280
As one who has taken the science approach to the oil (synthetic/petro) question. On my 99 SC400, I ran a series of tests with Mobil 1, Amsoil and Castrol GTX. After each run, I drained the oil and had it analyzed by a lab in Oakland , CA.
Castrol GTX is BAD after 5000 miles, thus 3000 would not be a bad change interval.
Mobil 1 could run for 10K miles and still be considered serviceable but with a low level of additives. Amsoil could be run for the same 10K miles but had a higher level of additives remaining. Both synths had a TBN (total base number) indicating serviceable oil. Both synths had lower metal particle counts than the Castrol.
My engine had very balanced compression (+/- 2 PSI) on all eight cylinders and burned no appreciable oil for all these tests and it's life.
Here is my conclusion. Synthetics pay off for long drain cycles (10K miles). The Amsoil method of changing the filter and adding a quart maintains the anti-wear additives BUT if there is a problem, like fuel / coolant dilution, the problem is compounded by not doing a complete drain.
I run a pure synth of wide viscosity range (10-50) and do a complete change on 10K miles PLUS use a very good filter (not FRAM).