Im assuming that in Guyana u get temperatures similar to us, ie constantly in the mid 30s in summer... yes that will affect your oil pressure, as if the oil thins down to much at certain temperatures then ull loose oil flow and oil pressure.. suggest u go for a fully synthethic 15w-50 oil.
As for just polishing a head its not really worth the hassle, might as well port it a bit once its off.. dont forget to leave the intake slightly rough
kon