Short question... long reply. ;-)
Ken,
Few things to remember when going to a synthetic oil.
The original oil has left a nice shellac on the inside
of the engine, quite often this will help in keeping
every thing sealed up and leak free. Synthetic has a
heavy amount of detergent, that will -over time- remove
the shellac build up. In my case the front cover never
leaked, but started seeping after a few months of pure
synthetic oil use.
The rope seals on the rear crankshaft. Majority of the
roadsters out driving around have a seal that will slightly
drip. Sometimes that will become a major leak with the
addition of synthetic oil.
Sometimes is the real seal is leaking badly on and older
engine, the real cause of the problem is excessive crankcase
blowby from worn out ring. In that case I'd almost recommend
keeping 20W-50 in the engine until it is rebuilt.
If the engine has been recently rebuilt, I run straight
non detergent oil for the first 1000 miles. Once the rings
are seated, and the engine feels good on power (usually in
the first 200 miles), then I'll switch to either 20W-50 or
Synthetic oil.
Pure synthetic is fine on a daily driver. If the car is
garaged for a period of time, the oil will tend to 'run
off' the camshaft and other surfaces. Aircraft engine
builders discovered this problem long before we did, as
many aircraft will sit for extended periods. Rapid camshaft
wear occurred on start up. Solution for Aircraft was a
blended oil.
Personally I like the Castrol Syntec (pure synthetic),
but still keep to 3,000 mile oil changes. If left in
for 7,500 miles you'll discover a varnish build up.
Best solution, in to use the BLENDED oil. Castrol sells
a blend, and I stick to my 3,000 mile oil & filter
routine.
Once again if it is an original engine with some mileage
on it, stick to the straight 20W-50. If it has been rebuilt
less than 10,000 miles ago, then switch to the blend.
Cheers,
Tom
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