Welcome to the site!
First - please be ready to take this in:
THERE IS NO SUCH THING AS A "BEST" OIL. There are many good ones, but there is no "best" one-size-fits-all lube, despite what marketing hype and rhetoric would suggest. There are always going to be some that are "better" for a particular set of circumstances, and those should be the ones you seek.
Next, there is some information you need to digest so that you can understand some of the choices you are about to make.
Synthetics are excellent products, but like anything else, there at always beneifts and limitations.
You really need to consider several things before you commit to any product, as one of but many decisions in an overall maintenance program.
- OCI duration (Oil change interval)
- compliance with warranty provisions
- operating environment
- operational conditions
- lube choice availability
- lube choice costs
- filter selection (complimentary to the lube choice, depending upon OCI)
etc, etc.
Probably the greatest advantage to synthetics is going to be greatly extended OCI durations. However, if you are unable or unwilling to commit to such, then there is very little chance you'd ever make the choice pay for itself. And, while synthetics do offer great low-temp performance for starting, today's direct-injection common-rail diesels don't make that as much of a factor as the old IDI motors with no intake-heater. They do "flow" better at uber-cold temps, but that rarely manifests into actual differences in wear reduction; the evidnece just does not support such a conclusion.
Spend some time reading in the diesel UOA section, and review many of the Cummins units. They perform very well, pretty much regardless of what oil is used (brand/grade/base stock). There is an excellent example of a guy who runs VPB (Valvoline Premium Blue) up to 20k miles, and gets great results. That is not in any way an assurance that you'll get the same results, but it is an indication that synthetics are not required for good wear protection. Excellent wear protection is not an attribute exclusive to synthetics.
In short, to make any product pay for itself, it needs to meet performance standards, and then provide that protection over a duration that can fiscally justify its use. When synthetics cost 2x-3x more money than a conventional oil, they have to be run out two or three times futher, just to "break even" on the cost, relative to the protection they provide.
If you do choose to run synthetics, there are many good choices. If you choose Amsoil, please consider using one of our site sponsors; support those who support us.