Oil has 4 levels - Good (green), Normal (light green), Worn (orange) and Bad (red).
- First level last for ~250km. The distance is not the same for all cars. Some can do 300km while others can only do 200km.
- Second level is betweem ~200-600km. Slighty loss of performance.
- Third level is between ~600-1500km. Gradual loss of HP and Torque.
- Finally, after ~1500km the car reaches the final level and it stays there until you bring it to GT Auto for an oil change.
I did a lot of races in Tokyo with the Cayman GT4, Corvette C7, R33 and Evolution V. Ths is my experience with them:
- Good oil provided the best lap times. I did sub-27 minutes with the C7, R33 and Evo.
- Normal oil was still good enough for a win, but I was about 1-2 seconds slower.
- Worn oil is enough to see a difference, especially when I couldn't reach top speed at the main straight of Tokyo. I was 4-5 seconds slower. Also, I do think there's a change in handling, but not a "direct" change. More like not getting used to a slower acceleration and speed.
- Bad oil is 7-10 seconds slower. Be a responsible owner and don't let you car reach this stage.
I did recorded my distances with both road and racing Vettes. Note that in the first pic, 574km is when the car start losing HP and Torque. 2nd and 3rd pic shows that you can lose performance and still be on the same level of wear:
This is only a temporary loss of performance though.
Now the question is: Bad Oil speed up engine degradation?
After reaching 1.580km with my Corvette Gr.3 (last pic) I did A LOT of races with Bad Oil. Probably 1.000km. Engine and Chassi changed to Normal after 5.079km
Unfortunately I don't have another Gr.3 to compare, but I do have 3 road cars above 5000km: C7, R33 and Evo 5. It's any different?
- First car: R33. I did my best to maintain Oil levels on this car. Always running on Good or Normal. After 5.042km the Engine and Chassi changed to Normal level.
- Second car: Evolution V. This was the opposite. I probably used this car for more than 2.000km with Bad Oil. Engine and Chassi changed to Normal after 5.030km.
- Third Car: Corvette C7. A mix of the two cars above - not well maintained, but also not abandoned. Engine and Chassi changed to Normal after 5.070km.
The distance to have Engine and Chassi changing from Good to Normal seems to be "fixed" for each car. Well, maybe there's an impact driving with Bad Oil, but looking at those distances, I'd say they're not significant.
On GT5/6, you would see loss of performance after 10.000km or 15.000km, depending on what car you're driving.
In GT7, Engine and Chassi reach Normal level after 5.000km and based on Tomahawks I've seen here, Engine and Chassi reach Worn level after 20.000km and they finally reach Bad level after 30.000km.
Between 5.000km to 10.000km, the Engine Overhaul option at GT Auto restores the car performance completely like it's brand new. After 10.000km there's a permament loss, about 1 HP less. From this point, Engine Overhaul will not restore the car HP back to it's original values. You can only do that by buying a new engine from the tune shop (if it's available).
I don't know how many HP you can lose from a high mileage car, but on GT5/6, you need to drive more than 200.000km to see a total loss of 5%. 200.000 KILOMETERS. That is a lot!