Quick story on this topic. I have an early sixties Gravely Model L two wheel tractor with sickle bar mower, both bought used, no idea on history. I serviced with Mobil "super" conventional 10w-40 2.5 qts. and new Fram orange filter.Good idling oil pressure after service. Mower serviced too with correct lube and sharpened. I was cutting heavy weeds, goldenrod ect. in about 85-90 degree air temps.Tough going but I was not pushing the machine too hard. I stopped for lunch and thought I'd better check the oil. Gauge showed very low idle oil pressure,"red zone", and would not come up at higher rpms.
Engine hot by observation, no noises.Engine fan works and cooling shrouds in place, fins clean.
Changed oil the next weekend to Delo 15-40 HDEO and Wix 1515 oil filter. Oil pressure came right back up, no engine noise or ill effects even when warm. I have not cut with it yet but I think the HDEO solved the issue. Not sure if straight 30 would have worked as well.