Help AGAIN, oil light etc....

Right lads, am off work tomorrow    so I'm gona have a bash at checking the timing, BUT the jobs are starting to mount and I'm getting peeved off by the day….  I done another oil change the other day and used some oil from Halfrauds !! it didn't say if it was semi or anything but it had on "for early injections from 83 to early 90's" it was in a 5ltr bronze coloured container, anyway since then after a few miles the oil light flickers off and on at low revs, plus the buzzer sound has been on and off,  I've been reading lots of threads on here and they all seem to point to the wiring or sensors !! can anyone shed some light on it… my head is proper done in now.   I'm thinking it's down to the oil being ShXt, there is an interesting thread on here about dropping the oil and at the same time removing the sump and also the oil pump/pickup pipe/filter and checking it's not blocked, I think I'll give this a try tomorrow also and maybe some decent oil, when the head went a few month back it went to the garage and six ltrs of crap was drained, so I'm thinking there maybe some kind of blockage.
Just want some advice about the light and oil etc… sorry to be a pain and I imagine most people on here have felt like you take a step forward and 20 back… going nowhere. :dontknow:
Cheers lad.

This is a continue.. well kind of the following thread New Cylinder head :o(