Mobil 1 synthetic gear oil is NOT rated for the manual transmissions in Fiero. Or most cars for that matter. (You didn't say if it was motor or gear.)
Syncromesh is made for both the 5 speeds used in Fiero. (Read the stick article in my cave.)
You may need work on the cables and shifter as well. They do wear over time. DON'T put oil based lubricants on plastic parts! They may feel better for awhile but any problems you had are likely to return and be worse.
If you need to lube plastic, first clean it with oil free cleaners and then use silicone or dry teflon on it. Oil based things will react baddly with many plastics/nylons and it will bring dirt into whatever you lubed.
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11-Sept-01, The day the world as we knew it ended.