Drive it forever: Synthetic oil should be no problem

Dear Mike: After a recent trip to two major auto parts dealers for oil, I realized synthetic oil is slowly replacing nonsynthetic on the display shelves. The problem is this: I’m getting conflicting information about synthetic oil. One store attendant told me I shouldn’t go back to regular oil after changing to synthetic. Then he said it would be OK to add a quart of regular oil to synthetic, if I needed to top up. Another clerk said I should never mix the two. At a different store, the employee said it didn’t matter if I used synthetic and then later replaced it with regular oil.

A: Early synthetics got a bad reputation for leaking. This was because, despite the claims of the oil manufacturers, the seal-swelling characteristics of the new synthetics were different from those of the mineral oils they replaced. If the seal-swell rate was lower, the seals shrank and oil leaked from crankshaft seals and rocker cover seals. If the rate was higher, the seals swelled a little extra and the engine was tight. Then if the owner changed back to mineral oil, or added a quart when no synthetic was to be had, things got really bad.

Fortunately, the situation has improved: You should have no problem switching back and forth. Adding a quart of mineral oil to a crankcase full of synthetic will be fine. Read the fine print — a lot of the “synthetics” on the market are blends containing a substantial proportion of mineral oil.

Dear Mike: I have a 1997 Cadillac STS with 172,850 miles on it, and it’s got a slow oil leak. The mechanic at the dealership says it is a high-pressure leak since it’s not noticeable, but I lose about a quart of oil every 3,000 miles or 4,000 miles. What are the risks of a major problem here and the probability of losing an engine?

A: Wait, is this a leak or not? Is there oil dripping on the driveway? Or is the oil level in the engine just dropping? If there is external evidence of a leak, then you should be able to chase it, even if the mechanic has to use techniques like fluorescent dye and ultraviolet light to find it.

One quart every 3,000 miles sounds like normal oil consumption — and, at 172,000 miles, very minor oil consumption. And don’t let this mechanic sell you any expensive repairs.

Dear Mike: I have a 1987 Mazda B-2000 pickup with a Japanese replacement engine that I put in 15,000 miles ago. This pickup has about five miles of vacuum lines with the original carburetor. My problem is that it runs fine during warm weather, but when it gets to about freezing with some humidity, it starts and runs fine for about 20 minutes. It then starts to stumble as if the choke weren’t opening (it is). If I shut off the truck and let it sit, oftentimes this takes care of the problem.

One thing I have noticed since I put the engine in (I did put in a new water pump and thermostat) is that it never warms up to operating temperature on the gauge. But the heater and defroster work fine. Any help or suggestions would be greatly appreciated, since I don’t want to just jump in and start replacing things.

A: It’s carburetor icing, caused by moisture in the air. As intake air enters the carburetor and speeds up, its temperature drops. If the air is near the freezing point, the moisture will turn to ice where the temperature drops — right inside the carburetor, where it makes your engine run poorly. Shut down the engine, and it melts within a minute or two. For the rest of the day, the carburetor is warm enough to keep ice from forming.

Any vehicle with a carburetor has a stove-like arrangement on the exhaust manifold to preheat intake air under these conditions. Yours is rusted out.