Yeah, I've been looking at the STP oil in Australia. I'm pretty sure it will be different to the STP oil you get in North America. Maybe it's like a franchise and a local blender buys the rights to use the name. The name STP has been around for a while, so that has to be worth something.
You had a mineral 5W30 HM that was SN rated and $24 (USD) for 5 qt (4.73L).
Over here in Oz, Shannow showed a full synthetic 5W30 that's rated SN, A3/B4, BMW LL-01 & Porsche A40 for $55 AUD (or $38.50 USD) for 5L. But right now SCA is having 15% off most STP oils, so expect to pay more like $47 AUD ( or $33 USD).
The local STP semi-synthetic 10W40, SN A3/B4, is $40 ($28 USD) for 5 L. But right now is on sale at $34 ( $24 USD).
To me STP is one of the good second-tier products we get along with others like Nulon and Chief oil. I'm sure they are fine, but I've never purchased any. The reason being, their regular price is better than name brand oils (Castrol, Mobil, Valvoline, Shell & Penrite) but we regularly see the name brands at 40 to 50% off, but the second tier stuff is mostly full price or at most 15 to 20% off. You get more oil for your money, if you plan ahead and wait for the big names to go on sale. I recently picked up 5L of Castrol GTX with an oil filter for $20 AUD (or $14 USD). That's a good price.
If they offered me STP full synthetic at 40% off a few times per year, I would probably give it a go. But as it stands, I tend to stick to the name brands at about half price.
Still, I like having a big selection on the shelves, it looks good and keeps the big players honest.