Yep my tranny will pop out of second when not under a load, I just put a different one in last fall & this one does the same thing? Anybody else?? 55 crown Vic 292 3 speed overdrive. 

Most commonly the synchro for that gear is worn and causes your symptom. Could also be the linkage is out of adjustment, but that usually causes hard shifting along with popping out of gear. I would check adjustment and oil level 1st. If you have a shop manual it should explain the adjustment procedure. And don't use any kind of hypoid (differential) oil in the trans; the additives that make it hypoid attack yellow metals, i.e., the synchronizers.

Had a tranny guy put new gaskets & seals in it when I got it, he said it looked great inside. Oil level is good, I do have a shop manual & will look into that. I think he put 80/90 in it - at least that's what it smells like to me - seems to be shifting harder lately, what gear lube should I be running??    

My '55 Merc manual calls for 80 wt, 5 lbs or 5 pints w/OD so your 80/90 should work. The proper oil is GL-1. It is non-hypoid and commonly used in agricultural equipment transmissions. NAPA has it or can get it, Tractor Supply carries it and so do CO-OP farm stores.
Also have look at the 2 bushings where the shift levers attach to the linkage at the bottom of the column. They're mounted in rubber and when they're worn out they cause sloppy shifting. You can get replacements from the usual suspect classic parts suppliers.

80-90 is fine. Should be GL4, not GL5 which as stated earlier is for diferentials. Its a little hard to find. Finally had to order it on line to get it. Do not use GL4 in a diferental.

Ok cool, I will ask trans guy what he put in there, Thanks!!

Oops! You're right! It's GL-4, not GL-1.

I should add that when you add oil fill the trans section 1st, then top up the OD. Don't rely on the passage between the trans and OD sections to lube the OD. If you'll notice, the OD filler is a couple of inches higher than the trans filler.

GL-1 is right Dobie. Fifth Avenue doesn't recommend anything rated higher than that. They sell the oil but it's $30 a gallon. I got my oil from NAPA for less than half that price. I still have a partial jug left if you need the part number.

Durham Missouri

I have two OD trans, R11 in my 57 and a R10 in my 60 truck. I have the Borg Warner manual for the OD units and they specify using 30W oil in the OD unit. I have never done that, I've always used 90W gear oil which is about the equivalent of 50W motor oil.. Gear oil and motor oil viscosity are not the same.