We have had a few customers report that their Lubricheck used to work fine, but something changed, and now the readings are inaccurate.
When those units came back for testing under warranty, we determined that some users are holding down the test button, before turning on the power switch. In this condition, if the button is held down long enough, the unit will re-callibrate to a meaningless oil value.
So, the message here is, don't do that! Only press the test button briefly, after the power switch is turned on, and after the oil sample has been added to the sensor cup.