Medium alkyds vs Long alkyds

[BFZHA]

Hello,
For the coating of our pipes we have specified long alkyds paint as a primer. The paint Vendor suggests the use of medium alkyds because it is better than long alkyds paint.
After a little search we found that as the oil length of the resin increases, generally the viscosity decreases, and the hardness of the film decreases, film flexibility increases and water resistance is reduced.

Pleas, could you advise if the paint Vendor suggestion is correct?

Thanks