•Q. Bonehead move on my part: While changing the oil for the first time in my 2005 Toyota Sienna van, I could not locate what looked like a normal oil drain plug. So, I ended up draining out the transmission fluid and then overfilling the motor oil. I did not realize my mistake until I was about two miles down the road, wondering about the rough gear shifting. I immediately returned to my shop, added transmission fluid and drained/refilled all the motor oil. I managed to drive the car a total of four miles and up to 55 mph with no transmission fluid. There does not appear to be anything wrong with the transmission, as it now shifts smoothly. (1) What damage could I have done to the transmission? (2) What damage might I have done to the engine by over-filling the oil? Thanks.•

TOM: Congratulations. You are now eligible to join the Bonehead Support Group. Send your application to my brother. He’s President for Life.

RAY: Gee, I’m surprised you even got the van to move at all if you really drained all of the transmission fluid. Normally, it’ll just sit there and refuse to shift into any gear if the transmission is empty.

TOM: My brother would prefer you not ask him how he knows that!

RAY: So, I’m guessing you didn’t drain out all of it. After all, it’s pretty near impossible to completely empty the transmission just by opening the drain plug. A good amount of fluid inevitably remains behind in the torque converter and the valve body. You obviously still had enough in there to move the car.

TOM: The answer to your question is: It’s impossible to know exactly what damage you did. Obviously, you didn’t do catastrophic damage, because the van appears to be running fine — at least from the point of view of a guy who doesn’t know what an oil drain plug looks like. That’s good news. What you don’t know is what less-significant damage you did that will simply shorten the life of the transmission or engine.

RAY: In the case of the transmission, the fluid provides lubrication. Moving parts operating with minimal to no lubrication obviously will wear down faster, and their lives will be shorter. How much shorter? I couldn’t even guess without taking apart the transmission (perhaps someone will be doing that very soon!). You could have problems in 10,000 miles, or the rest of the van could disintegrate 100,000 miles from now, before you ever noticed a transmission issue.

TOM: I’m less worried about the engine oil. Overfilling the crankcase can lead to “foaming,” where the crankshaft whips up the oil like a nice meringue. And compared with engine oil, foam is a poor lubricant. But it’s better than no engine oil. And you drove a relatively short distance.

RAY: But the same is true for the engine. Theoretically, there could be internal damage that will emerge in 10,000 or 20,000 or 30,000 miles. But I’m encouraged that the oil light didn’t come on (it didn’t, did it?). TOM: So your best option is to keep an eye on things, and hope you get away with it. But if you notice oil consumption, blue smoke coming out of the tailpipe or rough shifting, swing by Bonehead Toyota and pick up a brochure for the 2014s. Good luck.

•Q. My husband’s latest great idea is to convert our 2001 Ford F-150 truck from gas to diesel. I would very much like to know if this is worth the time and money to do. The truck currently has more than 100,000 miles and is driven maybe a few times a month, but he is convinced that this is a good idea. I can only imagine what it would cost to pay a mechanic to do the work. Can you help by providing me with some straightforward and unbiased advice? I’m counting on you to help burst the bubble.•

RAY: It’s actually very easy to drop a diesel engine into a Ford F-150 — if you happen to be a Ford factory!

TOM: For the rest of us, it’s much more of a project. So perhaps doing the math will make your decision easier.

RAY: If he’s going to buy a new diesel engine for this truck, he’s looking at a $10,000 expense to get started. Then he’ll need a matching transmission to go with it.

TOM: So let’s say he buys a used diesel engine/transmission combination. That’ll run him about $6 grand. Plus shipping. Add another $4,000 in labor and other parts, and let’s say it’s a $10,000 project.

RAY: But then you have to look at all the money you’ll save once you have the diesel truck. The cost of fuel obviously varies, but these days, diesel costs about 50 cents more per gallon than gasoline. And let’s say — for this calculation — you get 20 percent better mileage with a diesel.

TOM: So, instead of the 15 miles per gallon you get now, you’ll be getting 18. You say you drive the truck only a couple of times a month. So I’m going to guess you drive this thing 300 miles a month, or 3,600 miles a year.

RAY: If gasoline costs $3.50 a gallon and diesel costs $4 a gallon, your annual fuel cost for the gasoline truck is $840.

TOM: And for the diesel, it’s $800. So your annual savings will be $40.

RAY: That means you’ll recoup your original $10,000 investment in only ... 250 years.

TOM: So it’s a close call. I wish you guys luck as you struggle over this very difficult decision!

Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.