Back in January, I noticed that the apparent viscosity was dramatically different between two identical 5-qt jugs of Castrol 20W50 oil.

I wrote to a friend at ARCO/BP (parent company of Castrol) who forwarded my message to a tribologist at BP.

The tribologist answered me and was very polite and interested.

It happened that I still had several ounces of oil in each of the two jugs, so I sent the jugs to the tribologist for analysis. I did not bother to label the jugs. It was immediately obvious if you unscrewed the lids and looked inside while sloshing the oil around which was the "thicker" oil, so there was no need to mark the jugs.

After several months, I got a message back that the two oils were both "within blending tolerances".

Apparently, blending tolerance for 20W50 is anything that will pour out of a jug. Makes you wonder why they bother to put numbers on the jug.

I rather suspect that he found a problem and took action to prevent a recurrence, but he will never admit in writing that there was a problem, for fear of opening up the company to a slew of lawsuits for every engine that has failed while using that oil in the past several years.

I still have not decided whether, after 35 years and well over a million miles spread across a half-dozen vehicles, it is time to change oils. Two of the vehicles in the driveway right now have over a quarter-million miles each on them using Castrol 20W50 all the way.

I have never had a mechanical problem that could be attributed to oil, other than two 5-speed trannys that failed, each at about 175k, with synthetic gear lube in them. I don't use synthetic gear lube any more.