Q: My last two oil changes were with synthetic oil, but they mistakenly put conventional in this time. They offered to drain it, but I said, how about make it good next time. Any problems switching over?

A: There's no problem switching from conventional to synthetic oil or vice versa. No undue wear or damage will result. In fact, semi-synthetic oil is a blend of the two.

Time for new gaskets

Q: I have a 1991 Pontiac Firebird with a 305 CID, V-8 engine. It has an exhaust manifold leak. Both manifolds appear to be heavily rusted. Should I replace just the gaskets or the gaskets and manifold? Should I have both manifolds repaired or replaced? Is there any other work you would recommend doing, as well?

A: Unless the manifolds are cracked or warped, and they likely are not, just replace the gaskets. Don't worry about the rust. The cast iron exhaust manifolds on older cars develop surface rust from almost the moment you drive the car off the lot.

Starting issues

Q: I love my 1992 Dodge Dakota that in its 33 years has racked up only 125,000 miles. Occasionally when I start it, the revs will run up to about 3,500 and then quickly drop down to zero and the engine dies. Sometimes it will start OK but cough, stutter and backfire when the accelerator is pushed. A restart usually clears the problem, but sometimes it requires as many as five tries. Throttle body cleaning was suggested and that didn't help much. Do you have any ideas?

A: Clean or, better yet, replace the idle air control valve.

AWOL molding

Q: I'm having the same problem with my 2018 RAV4 that I had on my 2013 RAV4. The driver side molding on top comes loose easily, especially after a car wash. The dealer said it would cost more than $500 to replace both sides. Is this molding even needed? Or would super glue be an option?

A: I rely on 3M automotive weatherstrip adhesive.

Bob Weber is a writer, mechanic and ASE-certified Master Automobile Technician. His writing has appeared in automotive trade publications, Consumer Guide and Consumers Digest. Send automotive questions along with name and town to motormouth.tribune@gmail.com.