Why would an engine leak oil only at high speed?

Question

Small 90s econobox, 4 cylinder. ~300k miles. Stickshift, so engine speed is always proportional to road speed at relevant freeway speeds.

Crossing the country recently, I observed extremely high oil consumption and leakage all over the engine. This was later traced to a valve cover gasket that hardened (ceased to be rubber-like). That was replaced. There is now no evidence of external leaks.

However, crossing the country back, I discovered the engine still had very high oil consumption - 1 quart every ~400 miles. Clearly an internal leak. No discernable "blue smoke". However, this only occurred in the states that allowed 75-80 mph cruising. In the states which allowed 65-70 mph cruising, oil consumption was much, much lower - ~1 qt per 1000+ miles.

So clearly, engine RPM has everything to do with it. Altitude does not; it did the same in eastern Nebraska (1000-2000') as Wyoming (5000-7000').

A possible factor is the engine being about 1 quart low on coolant for about 1000 miles. Added water was gulped up by the engine, so clearly the top of the cylinder head suffered uncovery; however, the coolant temp sensor in the head read normal values at all times (there's a coolant gauge and it works). Perhaps this accounts for the valve cover gasket losing its suppleness, and perhaps the new oil leak???

(despite the misadventure, the cooling system is tight as a drum. No leaks, proper overflow tank behavior.)

Which source of oil leak would be indicated by "only occurring at high speed" and "possibly being caused by overheat from low coolant"?

Answer 1

At high speeds the oil pressure is higher so it may very well leak more than at lower speeds.

Just because there is no visible "blue smoke" doesn't mean the oil is not being burned. The two usual causes are piston rings and/or valve guides. But due to your stated history of overheating, cylinder head warping or even engine block warping are possible as well.

The piston rings can be evaluated using a compression test as well as a leakdown test. Both are simple enough for the DIY mechanic provided you have the tools. Note that many auto parts shops rent tools including these.

Valve guides are a bit more difficult to test but here is a site that has some procedures to try:

Valve Guide Tests

Basically you're looking for telltale signs of oil burning during engine operations that are likely to cause oil to be drawn past the valve guides. Note that these tests generally can only prove this is the problem but can't rule it out. The fact that you have a hardened and failed valve cover gasket leads me to believe that your valve guide seals could very well be in the same condition.

At 300K miles and 30-some years old, the likelihood that this engine needs one or more of its internal oil sealing components replace seems quite likely to me.