The car is Toyota Trueno GT-Apex 1991. It has 236,000km on it. It needs
topping up with 1 liter of oil after approximately 3 to 4 hundred
kilometers driving.

Is there any way to deduce whether it's the valve guides, valve seals
or piston rings (or something else) without taking it apart?  Am hoping
it is something cheap and easy, but the high km's would suggest
otherwise?

There are no oil leaks on the ground, it doesn't seem to smoke under
normal driving conditions, but it does put out a lot of blue smoke at
higher rpm's (over 6000 rpm). There is some leakage of oil coming out
of the seals on the top of the head. On cold nights it smokes for 10ish
minutes after start-up, but is hard to tell what colour the smoke is.
(Is assumed it is the oil again).

Have gone from 15-40 to 20-50, but has had only 1 liter in it so far.
Would the thicker grade make a noticeable difference over time? How
about some kind of oil additive?