360ci

The worst thing you can do is park a vehicle for a prolonged period. This allows any gunk already floating around in the transmission to gel at the bottom of the pan. Once you start the vehicle and put it in gear the 'gunk' can clog up the valve body creating shifting problems. I'd take it in for a transmission flush with such low mileage, and with any luck that will clear it up and it shouldn't cost you more than $100. Oh, I'd also find a new mechanic, as he doesn't know much about transmissions.

Overdrive is a top gear that is less than 1:1, for example, if first gear is 3:1. second gear is 2:1 and third gear is 1:1 (also known as direct drive), overdrive would be 0.7:1
Overdrive saves additional wear and tear on drivetrain components, lowers noise and aids in fuel economy. If you have the option on the shifter, put it in 3rd gear and then drive on the highway. You can then shift into overdrive at any time, just take your foot off the throttle to relieve tension on the transmission. This will give you an idea of how the overdrive works, as most people don't know. Especially those who have never driven a manual transmission with a tachometer. A general rule of thumb in 4 speed auto gas vehicles is every gear is spaced to allow a 1000rpm gap. So at 60mph if you look at the tachometer, the engine will be turning over say, 2500 revolutions per minute (rpm). If you were in third gear at the same speed you'd be turning over approx 3500rpm and in second you'd be turning over 4500rpm.

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'99 Taurus 3.0L 2V Vulcan 145hp; 124,250m
'00 Durango R/T 360ci 300hp (modded); 112,800m
'06 Pontiac G6 GT 3.5L 220hp; 21,100m

Amsoil in all vehicles!