False Super Saiyan (疑似超サイヤ人, Giji Sūpā Saiya-jin) is a Saiyan transformation that Goku takes during his battle against Lord Slug in the movie Dragon Ball Z: Lord Slug.
Since the false super saiyan is basically like an incomplete SSJ transformation, I'd say that it's less than but closely approaching SSJ power, in general. However, Goku's false SSJ was probably significantly weaker than his full SSJ.
False Super Saiyan which temporarily appeared in Movie 4 “Lord Slug” is Not Canon. It was released before the Actual Super Saiyan and TOEI didn't have the original design and color of a Super Saiyan yet from Toriyama.
Super saiyan transformation is possible only through the rage. Vegeta goes super saiyan in Androids saga(before cell saga). He is transformed through rage on goku as goku surpassed him and became a super saiyan.
Prior to his fight with the Desert Bandit, Goku demonstrated himself to be highly capable in combat despite his age, but Yamcha outmatched him with ease. Even armed with his Power Pole, Goku couldn't hold up to Yamcha's blows, who hit him with his signature move, Wolf Fang Fist.
In Dragon Ball Super, Frieza gained the ability to sense Godly Ki after his training following his first resurrection as he could sense Goku's power in Super Saiyan Blue. Hit had the ability to sense Godly Ki, as he could sense Goku's power multiplying upon using Super Saiyan Blue Kaio-ken.
You can use it past the expiration date, just make sure it doesn't smell funny. Bread: Bread will keep in the freezer past the expiration date as long as you don't spot any mold. Check if it's mushy, moldy or has an “off” smell.