[MagicRat]

Personally I use the same regular oil on my project car for 2 reasons:

1. The project cars are old and relatively primitive, compared to modern cars, so they would benefit less from synthetic or more expensive oils. By 'primitive, I mean the engines run cooler, are more simple, have looser surface clearances and tolerances and fewer moving parts that todays multi valve, DOHC engines which run hot for emissions purposes.

2. I drive my project cars generally less often and more gently that the modern daily driver, so the oil is less stressed.

And yes, I do change the oil in my daily driver less often, based on mileage. This is not because I care about it less, but the fact that it is driven more often and for longer periods of time means there is less opportunity for condensation and other harmful volatile deposits to contaminate the oil and necessitate a change.