conventional or synthetic oil?

it shouldnt do no harm to it, i seen people do it, you might get a little power

with that many miles you might benifit from synthetic as it lubricates better and protects like 10x better than convensional

Unless you have been using synthetic all along I wouldn't. At 100k the seals are mostly dried out, that includes the valve seals which were never that great to begin with on this car. The rings on this car are extreme low tension as well and once again not that great. If you begin using synthetic now I suspect you will end up with a smoking, leaking car within a few thousand miles. Now before I am told that I am a synthetic basher, I sell and recommend Amsoil Synthetic Lubricants and use them exclusively in my personal vehicles. With your cars age and mileage I would stick with conventional or rebuild my engine and use synthetics.

Agree with Hevster1.. if you using conventional.. stick to that. Not really good to switch it up after a high mileage car. If synthetic was used from day 1, then it would be good. But switchin oils, avoid it.

i might be convinced to switch over to a synthetic as late as 60k miles, but for the most part i agree with the fact that the seals are probably mostly too dry for a switch at 100k. -and- most tercel owners don't take too good of care of them. they are mostly beaters. if you MUST try to switch, then try using about 1/2 qt of a synthetic oil and add it to the remainder of the amount of conventional oil to reach proper capacity. don't be tricked into buying the synth blends...they are only 10% synthetic anyway.