Up shifting suddenly very difficult

[kongjie]

I was halfway through my short, 20-minute commute home yesterday on my 2006 when I realized that shifting up was getting harder and harder. By the end of my ride I was actually leaning back for leverage.

It was a hot day, probably close to 100, and when I parked I saw that the oil was on the low side. So I topped it off, thinking that maybe it was a combination of the heat and lowish oil.

Well, this morning it was mostly the same for the first 10 minutes, then up shifting got a little bit easier, but it's still not normal. Down shifting seems normal; but to go up, I have to exert a good amount of force and even then I don't feel the level "click." Every time I shifted up, I wasn't at all sure I had even changed gears until I let out the clutch.

Should I wait and see if it gets any better, or is there something I should look at? It's got about 12,000 miles and I use synthetic oil, and it was changed about 2,000 miles ago.