I too run my 1.9 l at 5 deg BTDC, ever since I saw that the Euro spec for
that year '84 was 10 deg BTDC.    I know that the gas in USA may not be of
the same octane etc.
I also think the compression ratio is higher in Europe too.

I still maintain that it's the total advance that really matters, under load
at that.
Probably this is what should be checked with a timing light.    I admit I
havent done it though.        Idle timing can be where it wants to be.

Probably the ignition is retarded on US vehicles for emissions reasons and
some power is thus sacrificed.
If the engine doesn't ping under heavy loads, its probably safe.  Mine
doesn't.
I enjoy the little extra pick-up and I get 20 -21 mpg on regular gas with
a/c.   It doesnt seem to improve without a/c.     Most of my cruising is 70
-80 mph.

Cheers,  Steve Cotsford