Well, here is my take on it based on what I have been taught over the years about using synthetic.
If that motor has not been maintained right along with synthetic, and you are now going to start using it for the first time at 100K+, you are asking for trouble. The detergents in the synthetic oil will eat away at the build up in that 100K motor( which it WILL have if it has 100K on dino oil even with regular OC's ), which actually have sealed dried out gaskets and seals, and eventually lead to leaks.
As a rule it is generally accepted that 50-60K is about the point where you would start having to question the decision to use it if it hadn't been used previously. "IF" the engine had been maintained properly up to that point and did not use any oil then you would be okay. About the time you hit the 75K mark generally it is not considered wise to swap to synthetic in an engine that has never run it for the reason I give above.
If it were me I would not do it. Not saying 100% you will have problems but you very well could. I would be extremely cautious about doing it. I guess if you are aware of the maintenance history of the vehicle so you know it was cared for properly( ie; 3-5K OC's not 7-10K OC's ), AND THIS IS A BIG ONE, the engine does not use any oil now, you could give it a go. Just don't be too surprised if you have leaks within a few thousand miles as the synthetics eat away at the build up that is sealing it. If it uses oil now, or has the slightest of leaks, it will get much, much worse with synthetic.
I wouldn't run Mobil1 in that engine until you clean it out a little first. Check out bobistheoilguy.com forums. They swear by auto-rx. You run it for 1500 miles and it cleans out the gunk build up from dino oils over time. It's not super harsh like most engine flushes. Google it and they have instructions on what to do. Then you need to run at least another OCI of dino, Havoline is probably the best there is for dino. Since that car is over 100K I wouldn't switch to synthetic. It may give you more problems than its worth.
What I would do with that Celica is do the Auto-Rx treatment and then run 3-4k intervals of Havoline with a maintenance dose of LC20(1oz per quart) and possibly some Valvoline Synpower Oil Treatment (1oz per quart). Maybe just one or the other, I prefer LC20.
PS. I did this with my girlfriend's Ford ZX2 with 90,000 miles and it's running better now than it did when she bought it 4 years ago.