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yannagas : Only HP is boosted 5% by the oil change. The PP and torque amounts will not change in the equal proportion that HP does because they themselves are not calculated with a direct factor of HP.
As for your numbers they do not quite make sense to me (I suspect a typo or an error in recording them):
You started at 631 HP
You dropped to 594 HP
What does 617 HP and -23 HP mean?
Was the 594 HP the lowest amount: i.e. did you drive over 300 km (with the oil deterioration gauge activated)?
Anyways, the oil change boost is easy to test with upgrades. Just do the test in the forward direction: addition versus subtraction.
Take maximum HP after upgrades have been applied and multiply by 1.05: this should get you the same value as you get after the oil change.
I did all the power upgrades to the Diablo GT2, raising its HP to 1006 HP.
Predicted 5% HP boost: 1006 HP x 1.05 = 1056.3 HP.
When I performed the oil change on the car with the 1006 HP the result was 1057 HP.
So the oil change boost does apply to any equipped power tuning upgrades.
It was a good question,
yannagas,

so I will update the original post.
[For those who understand the HP rounding issue: the car was actually at something like 1006.2 to 1006.49 HP but PD didn't show decimal place.]
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