I recently drove from Oregon to San Francisco and back, and I drive a Honda Civic. My car was fine on the mountains between the states, although there are a few long stretches of steep grades that were not so much fun to drive.
In fact, I drove through California when I originally moved to Oregon, and my car (the same Honda Civic, though quite a bit newer) was fine even though I had all my worldly possessions in it.
Both drives were in August/September.
What I find worst about that stretch of interstate are actually the other drivers, particularly the trucks. Some of them do crazy enough things that I ended up white-knuckling a lot of the time.
The I-5 corridor in the Willamette Valley is pretty much farmland, which I think is what people think is so boring about it. South of the valley, it gets more mountainous and starts to look more like northern California. It can perhaps be a little dull, but it has the advantage of being an easy drive. There are also plenty of places to stop along the way, if you need to or want to. I find Salem a bit hard to navigate from I-5, because I don't go there that often and I-5 really passes through the edge of the city anyway. Eugene/Springfield is nice, and pretty accessible from the interstate. Roseburg is a big enough city that there are some places to eat. I've never personally stopped in Medford, but only because I prefer to stop in Ashland (and the two are very close together).
Personally, I find northern California far more dull than Oregon, and there are far fewer towns/cities to stop in for a rest or a meal. I think it's partially that I start thinking, since I'm in California, that I'm almost there. Except that, if you're going to the bay area or Sacramento, you've still got hours left to go.