It's a known effect of all amphetamines (to an extent all dopaminergic stimulants) and is not an indicator of any cuts or the purity of a batch. Even people taking purified pharmaceutical amphetamine (Adderall/Dexedrine) complain about oily skin and hair. Ir even happens to Ritalin/Concerta (methylphenidate) users too apparently, though less frequently.
It's likely from a combination of increased body temperature, increased sweating, vasoconstriction (blood vessels tightening up), and even changes in horomones. The combination causes a lot more oil to be produced in the skin for whatever reason.
The easy solution is to shower frequently with plenty of soap (or whatever, even scrubbing your skin (gently) with shampoo on a washcloth is better than plain water. And carry around some baby wipes or the like to frequently "freshen up" by removing any oils from hot spots (face [esp. nose, brow, and cheeks],, hands, armpits). Maybe wear some light deodorant/antiperspirant/cologne).
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Try staying in a room with a fan or AC to keep you cooler.
Also, reduce your dose.
