Regarding the "contamination of the test", I think Danny3.7 just means that Nissan may (I say may... you should ask them) believe that any tampering with components which are oil related could make them question whether you did something which may place their test at risk. If you open the dipstick, they could by stretch of imagination argue that you siphoned some of the oil out of the dipstick.
For this reason, when I worked at a shop part time in High School, it was a policy that after changing oil, we put some breakable blue sealant over the drain plug. Anytime someone touches or puts a wrench on the drain plug afterward, the sealant comes off. Thus, if someone accuses us of messing up their car and we see that the blue sealant is gone, we know they touched it after we did. Now checking the dipstick is totally different but same concept...
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