I will add some interesting info that I have found that may clear things up-
1) SOME (not all) cars hp/PP are different between v1.0 and v1.01 of the game- maybe these cars that had lower pp/hp in V1.0 would experience a break-in, haven't tested it yet although Tallulah Belle's post seems to point to these cars having a break-in increase which would make sense. I've seen several examples of different PP/Hp between the 2 versions of GT6, as an example check out the Acura integra Type R- starts at 190hp in v1.0 vs. 197hp in v1.01 (coincidentally that difference is the normal 3% break-in increase that GT5 had.
2) I think some of you are confused about how the oil works, everything I've experienced in GT6 shows it to be the same as GT5. To summarize, when you change the oil the oil state stars out in a "bonus phase" a 5% increase from normal power, that phase is finished by 189 miles (300km), then it is in a "nuetral phase" for 5000km (no power increase in this phase), then after 500km the oil light comes on and power starts to degrade as you drive more. The oil "state" is completely independent of the mileage the car has when you change it (however the engine overhaul state may be altering hp at the same time depending on car mileage, but that doesn't start till after 5000km)
I would say you may want to read this thread that fully explains it for GT5, it also talks about engine overhaul (which does an automatic oil change in addition to the overhaul):
https://www.gtplanet.net/forum/threads/engine-break-in-wear-overhaul-and-oil-change.182613/
3) As some mention there could be differences in running online vs offline for engine wear. In GT5, if you ran online with tire wear off, the car mileage would increase but no engine wear would happen (they get out of sync which was really annoying). All offline driving in GT5 caused engine wear and mileage increase.
4)As most probably know, you can not go by the hp figures listed for a car in the dealership, they will generally not match what the car starts at if you buy it. However, the PP numbers listed for a car in the dealership seem to match what you get after you purchase a car.
So when talking about whether you are seeing a change with break-in it would be good to list if its online with or without tire wear, or offline, and what version of the game you bought the car in, maybe also what version you ran during your break-in test.