There's 2 parts to your question:
- Can I switch to synthetic oil. Yes you can and you totally should. When I started doing my own oil changes 8 years ago, I switched all 4 vehicles to synthetic oil. All of them are double or triple the age of your Fit, and all have double or triple your mileage. Two of them started leaking a little bit, but it's manageable.
Here's what happens when you put synthetic oil in an engine that's had mineral oil in it. Mineral oil has less ability to keep particulates in suspension, so it will allow particulates to settle in corners and on rough edges. This usually helps seals and gasket that are cracked or leaking to remain sealed.
Synthetic oil has a much greater ability to keep particulates in suspension, so during your first synthetic oil change, the oil will be very dirty because it will pick up all these little particulates that have deposited for years because mineral oil was unable to keep them in suspension. This will keep your engine cleaner, but it may reveal some leaking seals that were previously patched by deposits. You should deal with those leaks, but they are likely to be manageable until you actually have a reason to get to those seals and gaskets.
In addition to this, synthetic is way better for your engine. One of my cars was unable to start if it sat overnight at -10C or colder without using an electric block heater, but since I switched to synthetic, it always starts fine, even as low as -30C.
- Should you switch back to mineral oil if there are leaks. If you get leaks, the synthetic oil isn't leaking out directly because of a magical property of it, it's really a seal or gasket that is not providing a 100% seal. Switching back to mineral oil will not stop the leaks, you will just be leaking a less expensive type of oil.
