DAP suggests oil based primer because it works better.
I have also brushed on several coats of boiled linseed oil to prime, ahead
of puttying. It fully saturates the wood, allowing the putty to take a
proper "set", and slow the drying/aging of the putty. Oil paints for this
purpose are not being phased out, and will likely be with us for a long
time. Boiled Linseed is not particularly toxic - comes from pressed
flaxseed - and is a great wood preservative when used on porous wood, under
a coat of oil finish paint, or oil based putty.

I agree with Roger, use boiled linseed oil to prime the muntins before
glazing with DAP 33, that's what I do. The "long oil" or "penetrating"
oil-based primers are linseed oil based, read the ingredients on the
can next time you're at the hardware store. The "fast-drying"
oil-based primers have no linseed oil, the penetrating oil-based
primers do have linseed oil.

Ken