I purchased a Westy with an EJ22 in 2016 that had been professionally converted at RMW in 2013. I would get very low oil pressure at hot idle that would occasionally trigger the light. All other aspects of the engine seemed fine. Running heavier weight oil definitely helped the issue, as I installed a gauge and monitored it closely in all driving conditions.
I still wasn’t happy with my pressures even running 10-40 weight, but I wasn’t ready to tear the engine apart to chase the cause of low oil pressure. So I opted for a stop gap. I purchased an STI oil pump from a 2003 STI and it bolted right up. These pumps push about twice the volume of the stock 1990 EJ22 oil pump; per tables of pump flow I found online by year and model type. That extra volume capacity resulted in approximately double the oil pressure in all conditions. I was able to switch back to 10-30 weight oil and now have good idle and running pressures, even when hot. When cold, I hit the 80 PSI bypass on the pump, but that hasn’t turned out to be an issue.
It’s now been 8 years, and the engine is still going strong. I monitor the oil condition during changes by sending samples to Blackstone. None of those reports have suggested there is abnormal wear occurring internally. So, I never explained why my oil pressure was so low, but my stop gap solution is now semi-permanent.
