Q: I own a 2007 Lincoln Navigator. I'm having a problem getting it into my garage due to its roof rack. I'm wondering if a roof rack from another vehicle, like the Cadillac Escalade, which sits flush to the vehicle, would work on my Navigator. What do you think? -- G.S., Chicago

A: G.S., I don't think you'll have any success trying to get the Cadillac Escalade's roof rack to mount up to your Navigator's roof. Does your local Lincoln dealer offer aftermarket roof racks? You may want to see if they do. If not, you might want to Google "roof racks for trucks" and check into some of the aftermarket units that are available. Maybe there's something out there for your Navigator with a lower profile than the original.

Q: I own a 2003 Cadillac DTS with 49,000 miles. I have changed the oil every 3,000 miles, but I'm always down about one quart of oil before I hit 2,000 miles. My local Cadillac dealer told me this is normal for my car's Northstar engine. A technician told me that's a common fault for the Northstar engine. I would like to know what you think. -- J.L., Chicago

A: J.L., according to Technical Service Bulletin (TSB) # 01-06-01-011D, the accepted rate of oil consumption for your vehicle is one quart in 2,000 miles. If your engine is consuming one quart of oil in less than 2,000 miles, you should bring attention to this TSB to your Cadillac dealer's service manager. There is a problem with the piston rings on this engine and they may require replacement. If the dealer doesn't appear to be willing to resolve this issue, contact Cadillac Customer Assistance at 1-800-458-8006.

Q: I own a 2001 Buick LeSabre with a 3.8-liter V-6 engine. The tachometer goes dead while driving every once in a while. It'll do it once every two or three weeks. Sometimes it won't do it for three months. What do you think could be wrong? -- J.M., Capron

A: J.M., apparently, there's a poor electrical connection in the engine speed signal circuit between the Powertrain Control Module (PCM) and Instrument Panel Cluster (IPC). Since the problem is so sporadic, it might be difficult for a technician to find. But if the technician is patient and wiggles the wiring harness between the PCM and the IPC, the problem may present itself. It's also possible that the poor connection is inside the IPC. But before any parts are replaced, I'd make sure the wiring and connectors along the way are not the source of the trouble. You should seek out a technician that specializes in electrical repairs.

Q: I own a 2005 Mazda 6 with a six-cylinder engine. The type of oil is supposed to be 5W20. I was wondering if I can use synthetic oil. Some people told me that if I use synthetic oil and if I go back to using regular oil I can blow the engine. Do you think it's safe to use synthetic? Do you think it's better? -- Louis

A: Louis, while synthetic oil is better for extreme driving conditions such as very hot or very cold operating temperatures, you'll be fine with non-synthetic oil as long as you change the oil at the recommended intervals. And no, you won't ruin your engine if you use synthetic and then switch back to a non-synthetic. The most important thing, though, is to use the proper viscosity and API ratings according to your vehicle manufacturer.

Questions for Ira Siegel? Call the Auto Adviser Hotline at 708/633-6839 or e-mail Autoadvisr@aol.comAutoAdviser: Ira Siegel is an Automotive Instructor and ASE Certified Master Auto Technician