Well, I've experienced both. My 84 Daytona Turbo Z has clean oil after 3K miles, but my 92 Daytona's synthetic oil is black about 500 miles after an oil change. Is something wrong? No. It still gets 32 mpg and runs flawlessly. There's just dirt that about 15 oil changes has not removed.
I was a proponent of Bosch O2 sensors, never had operational problems. But when I went to remove one, the socket rounded off the hex and it's stuck in the manifold. Reason is, the Bosch sensor is about 1/8" longer than other brands, and the socket won't bottom out enough to grab all of the hex. For that reason, I won't buy one again.
Also, the Bosch sensor has 4 long slots cut in the sensor portion. Denso and other brands have 16 stamped holes to expose more of the sensor to the exhaust, which should give better response, no matter how the sensor is oriented in the manifold.