I beg to differ

A couple of weeks ago Tom & Ray dismissed a kind lady who had called in to say she fixed a rough idle problem by changing the oil. Well, we have done the same. Our 3.5 liter Nissan Quest began to idle very badly and cut out often. Our mechanic checked everything out (everything OK, but no result) before changing the oil and (completely) fixing the problem. He has encountered it a few times and puts it down to dirty oil not allowing the variable valve timing to operate correctly. Does this explanation stack up? Changing the oil (did), very much, fix the problem.

VVT issues due to dirty oil are possible.
Do you remember what year was the lady’s car? Did it have VVT?
All the Car Talk shows are repeats. That show could have been from before the era of VVT.

“That show could have been from before the era of VVT.”

More than likely, it was from before the era of VVT.

Derek N Boulton–Are you aware that the Car Talk shows you are listening to could be from as long ago as 20 years?

Was it this call? This one was from the most recent show.

Beverly Onk (Fortuna, California) - 1981 Ford Van

Beverly’s van occasionally dies when she takes her foot off the gas and she has to re-start it. Last time this happened, the problem went away after she got an oil change. Tom and Ray say the oil change couldn’t affect this, unless bats that were living under the hood flew out when it was opened. Most likely, Beverly needs a tune-up.

I guess it is possible a car w/VVT could idle roughly if the VVT mechanisms was gummed up due to too long a interval between oil changes. But that explanation, while possible, seems an unlikely thing to actually happen, or at least be commonplace. What I notice in my Corolla after changing the oil is the engine sounds a little smoother running. But it doesn’t idle any better or worse.

In any event, the OP’s theory wouldn’t apply in the Car Talk callers case. No VVT in a 1981 Ford Van.

Edit: Ok, new theory. If a car idles significantly better after an oil change, doesn’t stall any more, my explanation is there’s less junk getting into the throttle body via the PCV valve. Ok, I admit it’s a pretty flimsy theory, but that’s my story and I’m sticking to it!

As an interesting side note my regular oil change shop for years, after oil change freaking noise out the ying yang, (video by request) turned out the old oil filter had been superceeded by a new one, they put one of the old part filters in and life was good. They noted it and do not get the new model oil filter. WTF? you decide.

A relative once had a van in the early '80s, started cranking slow when hot.
Checked bat voltage while cranking, was OK. Changed starter, no improvement.
While staring under the hood pondering what to do next I decided to check the oil.
It was really, really black and really, really thin. Didn’t fell slippery between the fingers, but not gritty either.
We changed the oil and the problem went away.

“Beverly’s van occasionally dies when she takes her foot off the gas and she has to re-start it.”

Could increased friction from exhausted oil pull the idle down?
Remember this is 1980s oil were talking about.
Knowing my relative the oil in that van could have been some cheapie stuff that didn’t meet API.

Although there are many possibilities we could think of where “old” oil could cause problems, especially with vvt (as it is a recommendation in the diagnostics for problems with this system) the cases are extremely rare, and in an auto mechanic point of view when someone tells you something like i changed my oil and and my car stopped dying" you automatically think well either the customer is crazy or the person who changed the oil also fixed something small the saw unhooked…

i have heard many cases, and try to give everyone the benefit of the doubt, as an auto mechanic of 15 years dealing with many different engines and a ton of vvt problems, i have never seen “old” cause a car to die.

Although in my own experience i chased a mazda vvt problem for a while, only to come to find out it was too much clearance in the bearings of the engine causing low oil pressure. so i suppose if you want to be technical this was caused by old oil, though it wasnt fixed by changing the oil, only by changing the bearings.