With the help of other amateur fungi enthusiasts, I have established that the most recognisable physical difference (for amateurs) between Suillus granulatus and Suillus luteus is the absence or presence of a ring or annulus.
Suillus granulatus is mychorrhizal with various exotic conifers, meaning that it forms a symbiotic relationship with pines. Both Suillus granulatus and Suillus luteus were introduced to Australia from Europe with exotic trees.
Caps of Suillus granulatus are convex (dome-shaped) at first, becoming broardly convex, then flattening, sometimes disfiguring. The cap is viscid (sticky or slimy), although, in my limited experience, not always. There should be no remnants of partial veil on the rim of the cap or on the stem.
Stem is white, speckled with brown, and is not always centrally placed. Pores are yellow, aging to a dull yellow, and flesh is white to pale yellow. Spore print is yellow-brown.
Suillus granulatus stem is sometimes off-centre
Caps can turn up with age
My sightings of Suillus granulatus
[This will be updated with new sightings]
Greta, NSW - under exotic pines - Mar, May.
Hanging Rock, NSW - on the edge of pine plantation - Apr.
Suillus granulatus on the edge of pine plantation at Hanging Rock. Note the collapsed fungus (front right) - these boletes, like many, collapse and decay quickly and are often infested with magots.