They have ways to make sure there is no influence and doing the oil change isn't that way... they fill up the oil as stated in the bulletin and put tamper evident tape over the dipstick and the drain plug... if the owner trys to remove either, it will leave a residue and ruin the tape. Without that, whether they did the oil change or not, an owner could technically drain a quart of oil from the block and intentionally fail the test.
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