if 20w50 used for long time then better not to change the oil at this point of time.
10w30 is recommended but since your car is old you should use 10w40.
Experimentally you can shift to 10w40 and monitor - if engine is not consuming oil then you can continue with 10w40 else switch back
Don't flush your engine however theoretically ceratic will be beneficial but i don't have any practical experience with such stuffs.
I heard and read many times that flushing an old engine is not a good idea due to following reason.
A good engine flush can help loosen deposits and dissolve sludge, returning your engine to like-new condition. However, in old engines with high miles, sludge may be the only barrier keeping oil from seeping through worn or cracked seals. Removing the sludge exposes the seals for what they really are – junk. Soon, your engine begins leaking oil, and you’re mind instantly associates the engine flush product with an oil leak.
In reality, the seals were already bad; the flush simply revealed their true condition.
source: http://blog.amsoil.com
