i had some bad lifter tick a few years ago while going to work (my 1.6TD hyd) and it was caused by lugging the engine at 1000 rpm (might have been lower actually) and then the lifter tick was very pronounced, and didn't go away. i pulled over... nothing obvious but the tick, so then i gave it the old italian tune up and bingo, noise gone

i'm sure it was oil pressure because the engine wasn't spinning very fast at the time...