Hello.
My name is Famine and I have compiled a full Used Car List for weeks 1-74 (that's days 1-518) which you can find elsewhere in this forum. And I feel that I am qualified to answer this.
First off, there are NO cars which appear for just 2 days.
Secondly, days 300-302 straddle two weeks (43 and 44).
Thirdly, there is NO Audi S4 of any variety in week 43, although there is a standard one in week 44, in the PAL version.
Fourthly, there is NO AutoZam AZ-1 in week 34 (which covers days 234-236) at all, in the PAL version.
Fifthly, only one totally fake car has ever appeared in GT - the Nike Car. Unless a Japanese S4 and a German AZ-1 exist in real life, it's unlikely they'll be in the game.
Finally, the Japanese cars are all super-duper-hyper-good in GT anyway - so why would PD try to make one better by making it part-German?
It looks like someone's spun you a line.
