What oil?

If it is 123K on the motor since rebuilt or original and is still running good, leave it alone and use 10w40. Unless the seals are bad, it probably doesnt need it. I would look into the chain, though, even though it is a double roller from the factory. If it runs good, it should work well with 10w40 and last for quite a while, considering oil pressure is good. The addition of hardened seats for sure helps in performance over the years.

These small blocks run for a long time with care.