I think 10 laps around the 'Ring is a bit too long especially with cars that are close to a thousand. I think 3 laps is reasonable, esp. with slower cars like that Model T, Mazda Carol 360, Subaru 360, the old Bug, and the Fiat 500.
I'll do the same thing, just like in GT2 (in fact I used Gameshark just to unlock the cars that are rare, and not available in the US-version) as what I did at the Test Course [consists with 1 warm up lap, and 3 main laps of each cars based on hp].