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?