In the early 1980's there were lots of articles published in the SAE journal (many from GM as they were switching most cars from 10W40 to 5W30) concerning cold start wear. Supposedly these were the longest, most complete... and most expensive test series that had ever been carried out up until that time.
Their lab testing.... using various radioactive isotopes on the pistons, cyl walls, etc....and oil in the one-time-pass mode were very interesting.
Cylinder wall wear is extremely high for the first 3 miles after a cold start, virtually no oil flies off the rod bearings to lubricate the cylinder walls. Cold thick oil trying to navigate a space of.003 inches (bearing clearance) results in only one drop of oil every several seconds, and it usually flies down, sideways, straight up into the piston.... and rarely onto the cylinder wall. Owners sitting looking happily at the 60 psi on their pressure gauge do not know that pressure is high only because no oil is flowing..... it is all going through the pressure relief valve.
When hot, the MUCH thinner oil is flying out of the rod bearings almost like a shower, and the pistons are hydroplaning on a nice layer of oil.
Interestingly the same test procedures showed no problems with bearing wear.... stating that bearings were well lubricated at startup and did not need additional oil except to carry off heat under high load condtions.....seemingly making a pre-oiler useless.
Remember the Ford money-saving idea (1974 to 1977) to not drill the additional "squirter hole" in the upper corner of the big end of the rod bearing?? This resulted in lots of engine failures even though it has been "thoroughly tested" but on hot engines. Nearly all the engine failures were in the northern states due to poor lubrication until after warmup..... extreme bore wear and scoring.
5 of my friends had their engines rebuilt under Fords secret extended warranty at about 40K miles - 2 Pintos, a Maverick, a Mercury and a Lincoln Continental.
Then, my 1978 Zephyr wagon lost the back lobes on the camshaft, they relied on oil flying from feed holes several lobes further forward..... and they starved during cold starts. This failure (as witnessed by warranty data) was also mostly a northern issue, and design changes were made to the camshaft, drill the feed holes further back.
Interestingly, I rebuilt the Zephyr engine at 100,000 miles.....10W40 the whole time and it was worn out, shot, extreme bore wear, guzzling oil.... and ran in on 5W30 after the rebuild. The rebuilt engine was still like new after an additional 150,000 miles.
Now, using 0W20 oils, both our commuter car and extreme duty tow vehicle have reached 300,000 miles with engines that seem absolutely like new...... so no reason for me to argue with the conclusion that thinner oils are MUCH better for cold starting and operation.
