what oil to use

Machine lifter bores?

>That it?
>Big job?
>Where would I get specs?
>Would changing heads be easier?

If you are asking if doing the actual work of changing a set of heads is an easier job then having your lifter bores worked for a roller set up... reworking the lifter bores is the more difficult.

The lifter bores are machined out then fitted with inserts (if it is done right). The reason is that you would now be using roller lifters and the tolerance specs for them are more precise.