There has been much writen on flaring, be sure to check the archives.
Flaring and harsh shifting are at opposite ends of the shift condition. Flaring refers to the increase in rpms which occurs during delays in proper shift overlap. Shifting in many cases requires the activation of one element after the release of another. Because these activities are not instantaneous the timing of these events requires the added element to start before the released element. Shift pressure (as affected by modulator pressure) is the only external way to affect the timing relationship (more pressure shortens all timing intervals).
The reason flaring comes about and the reason lubeguard helps is twofold. The amount of leakage from the seals of activating elements (clutch drums/band servos) is a great contributor to disturbing timing. The lubeguard type products softens seals and reduces internal leakage. The product also is a friction enhancer which alows older discs to effectively engage quicker.
Unfortunately the results of such products may be short lived. In the case of the early diesel (without modulator) I would suggest shortening the control linkage (delays shifts and increases shift pressure).. For those with modulators I would increase the pressure by a turn and go from there.
For those with harsh shifts, I would reduce modulator pressure as long as ALL shifts are firm.
Bosch Master, ASE Master, L1
33 years MB technician