This is by far the first topic I saw regarding Hyundai Xteer. I've started using it this year and just replaced my engine oil with it for more than a month now. Regarding with this Toyota engine seizing-up, this is NOT the fault of the engine oil, in fact the engine oil has done its job, in fact too well. Hyundai Xteer is Synthetic (Synthetic/100% synthetic depending on which you bought), and have better cleaning power than that of previous oil being used, this can cause debris that act as "seal" on OLD engines to get removed and can cause leakage. In this case, if there's no visible leak outside, it means that your oil is leaking inside the combustion chamber, which was being burned every time the car was being used. The sludge on the camshaft is due to the lack of oil because most of them were already burned. IF you have an old car and has been using full or semi synthetic oil then you changed to 100% synth (for example), you should have checked your dipstick once every 500km of use just to make sure that none is leaking, in this case, into the combustion chamber. This is more of a user's lack of maintenance knowledge instead of the oil being "bad".
Hyundai Xteer Ultra Protection 100% synth 5w30 (just to be specific) was the ones I used for my Ford Escape and it's pretty good. My previous oil was causing the gas pedal to vibrate, but now there's no vibration at all and the engine is running smoothly even on low RPM which the vibrations should be happening.
