farmall cub jumps out gear

Have a 1949 Farmall Cub. Use it around the house to cut grass plough gardens in the area. I have been having troubles with it jumping out of gear on hills! In 1st and in 2nd and sometimes in Reverse also. Is this due to the shifting handel being worn or the forks bent etc? I have looked inside the tranny a while back but really didn't see anything wrong with it,but of course i am no farmall mechanic.I have put in some Lucas 80-90 when i got the tractor,Any idea on this....?

Your jumping out of gear is not caused by the oil. The shift forks on your Cub are formed steel and bolt onto the shifting rods the shift lever move forward and backward. They come loose and don't completely move the the gears into full engagement. I think there's six short capscrews on the plate the shifter is attached to, remove those capscrews, carefully lift the shifter & plate and see if the shift forks are loose. Tighten the capscrews holding the forks if so.

You may have a bent fork too, or ground the gears into engagement too many times, or have a bearing or two getting worn and loose about ready to fall apart.

I've been around gear drive Cub Cadets for 55 years which uses the exact same components as your Cub.

If your shifting mechanism is all in good condition and this still comes out of gear probably the teeth on the gears involved are worn from grinding into gear. The teeth get chipped/worn so they have a small angle to them that, when power is applied, causes them to slide apart. I haven't seen this in a Cub specifically but in other transmissions. If it isn't a transmission with sliding gears then the "dog" teeth that engage get worn the same way from grinding and cause the same problem. If the shifting fork and shifting mechanism has to hold the gear in position under load it will wear out the shifting forks as they are not designed to hold gears under end thrust, just to move them. When in position they should not be moved by the power flowing through them which worn teeth can cause to happen. This wear can be very slight and difficult to see. It can look like a taper on the tooth from one end to the other. Or on "dogs" the engagement surface will be rounded off and then slightly tapered. Both cause end thrust to the gears which cause them to move and overcome the slight resistance of the shifting mechanism and them "pop" out of gear. You do want to make sure the shifter is working right and shifting it all the way into gear.