If you overfill, the oil as it expands when it's hot, gets pushed into the intake manifold trough the breather hoses. Then some of it burns in the cylinders and some of it stays inside the manifold...

 

My mechanic told me that the oil has to be very hot to make accurate readings. One way to decide is feel on RHS of car front- it needs to be very hot to touch.
Problem is i rarely get the engine that hot!

 

My 95 C4 behaves very similar. Its sweet spot seems to be between the bottom and first twist on the dip stick. If I fill it from half to full on the dip stick it seems to go thru that amount very quickly. My check/fill routine is pretty consistent and over the past two years I've come to accept that this is "normal" for now.

I'd be interested in before/after behavior from someone who's had an engine rebuild done. Did the sweet spot "move"? When I get around to a rebuild I'll try to keep this in mind and report back.

 

Mine seemed to move - once I rebuilt the Targa engine, and topped the oil say to the 2/3 mark on medium warm, it just stayed there - virtually no consumption. However, the engine was consuming oil like a chinese cook before that, so its hard to compare, but I did know that it consumer more from 2/3 to 1/3 on the dipstick before it slowed down.

 

Yes, I wonder about this.

My working hypothesis is that when I check and add oil, the car is never up to temperature, even though I ordinarily do it after half an hour or so at ~75mph. This results in me overfilling unless I compensate by lowering the fill point to ~1/8" from the dipstick's tip.

Do thermostats fail in this way? Test?

 

Yes, our thermostats fail. There was a recent post about this, I'll try to find it.

Our thermostats are very dynamic. They open and close throughout your normal drive.

He is what I noramlly see:

1) Cold start, therm closed, needle sitting on bottom marker

2) After a couple miles of local driving to the highway, the needle moves off the bottom marker.

3) After about 6-8 miles of highway driving, 65-70 MPH, approx 2,400 RPM, 70 degrees air temp, the oil temp needle starts to move just slightly above (a needle's width) the second mark (2nd marker above the bottle mark). This is when the therm opens.

4) You can see the oil temp drop to about 1-2 needle widths below the second mark, as cold/cool oil now rushes into the system

5) Then the needle will come up about one width

6) I can then see the needle vacillate to one width above the mark to one width below the mark, as the therm opens and closes, to moderate the oil temp during a highway drive

I'd venture a guess, that if you don't see the needle drop in step #3 above, your therm is stuck in the open position. Also, if you don't see the vacillation I noted, it's another indication yours is stuck in the open position.

 

Some good info in here:

https://rennlist.com/forums/993-foru...-change-2.html