DonCherry

1。此时跑高速是否很危险，
No, it's OK. But don't stay too long in Ontario or US, they have more strict pollution law.

2。该如何处理这个问题？万分感谢。
No choice, buy more engine oil, add reguarly or rebuild engine(not worth it). Most Chrysler and Hyndai have this problem not matter how you maintain.

DonCherry

If the car doesn't leak oil, and the quantity is going down, then the engine must be burning it OR leak into coolant.  You don't have to be leaving a big black cloud to be burning oil.  I would expect a small amount of consumption. This may sound weird, but it may shed some light.  

Take a small amount of cloth, or some other fabric that will not restrict airflow, and cover the end of your tailpipe with it (just one layer, don't restrict airflow), then take your car for a ride at speeds you normally commute at. After the ride, look at the cloth.  Is there oil? Also, check your coolant for signs of possible oil contamination. Check your water system for ontamination...you 'll need a new engine head. Otherwise you're burning it.