I've used it in 1st (accidently!) without any troubles. The reason that the OD isn't used in lower gears is that if the shaft isn't turning fast enough, there won't be enough hydraulic pressure to shift or maintain the high speed position. That depends in part on the condition of the o-rings and the viscosity of the fluid that you are using. The electric solenoid, when engaged, routes the fluid to the pistons. The pressure that is created is used to shift from low to high. It drives an eccentric cam that pumps a little hydraulic piston in the OD. Last thing before watching fireworks is that the hydraulic pressure is created by the input shaft RPM. It would take a little engineering to find the right spot.

If you are really enterprising, you could mount a reverse switch to the end of your transmission. Anytime the clutch is engaged, the OD shifts to low. You could also put it on the clutch, which would be more useful, I think. Anytime the brake is engaged, the OD shifts back to low. I may add a brake or clutch over-ride as well. Maximum Overdrive has a schematic for a brake override to protect the OD. I have a dash light that reminds me when it is on. There are a lot of warnings about putting them into reverse, so I'm going out of my way to see that it doesn't happen. I have two units and both seem pretty solid. I've never heard of anyone blowing one up.

It is possible that the clutch material may wear out. The gears are pretty much bullet proof, unless they are sheared off by a load, and it would have to be a really big load. There just isn't much to these units, and not much to go wrong unless you put the seals in incorrectly-generally, either the electric solenoid goes bad, or the hydrauling unit needs the o-ring kit. If you get the o-ring kit, you'll also get some very nice images of how to rebuild the OD.

Laycock j type overdrive manual