The head bolts lose their clamping tension over time.
The picture from another thread shows the ten head bolt holes in the block.
From the picture, you can see the oblong, water jacket holes.
The big amoeba looking hole between cylinder two and three is the oil return channel.
Where is drops down, just above it is the oil pressure (feed) port for the cylinder head.
Just above that is a water jacket hole.
If the cylinder head bolts get loose, oil pressure at 45 psi (or less) can get pumped into he water jacket.
You seeing oil in the overfill bottle indicates you have a head gasket that has a seep leak.
You might be able to retorque the head bolts to 65-70 Ft-lbs and avoid gasket replacement if you act quickly.
If you retorque the head, oil that is being diverted into the water jacket (with a resultant loss of pressure) might get into the head, and stop your lifters from ticking due to inadequate pressure.