Are you sure it's a sugary syrup? That would be quite unusual, I've never seen that on the underside of leaves.

I did taste it, it tastes kind of bittersweet. But dont count on my tastebuds too much :D

Anyway, undersides of leaves are sticky, some more than others. And its distributed equally on the whole underside of the leaves. Except the stalk.

It does seem strange. How high is the relative humidity around it? Do the leaves still look healthy?

Relative humidity is too low, around 50% now at the springtime. In the summer it raises to something 70%. Thats why I mist my plants daily.

Leaves do look healthy to me, I cant find anything else that would lead to a diagnosis. My only guess is that it is something related to watering. Shock or something that leades to cell eruption?

I have the exact same problem my mango is about 5 months old

It is the same with my mango, it is 3 months old. There are two sets of leaves. First he got set of 6 leaves, and afterwards, he got set of 4 new leaves. Only that first set of 6 leaves is sticky from underside. But the mango looks perfectly healthy.

Look for scale.

I had the same problem and it was

Could it be that you have aphids?

They secrete a sticky sugary substance for their "friends" the ants because of their symbiotic natural relationship.

any conclusion on the stickiness on underside of leaves? i have a 9 month old plant; 12" tall; lives indoors; 25 leaves altogeher;within last two days, the top 10 leaves have an even coat of stickness on underside; nothing visible except those leaves are not as green as the others. should the sticky leaves be washed with soap and water?