Engine trouble- Plummeting oil pressure

My 2 cents, new engines like a 5.7, 6.1 Hemi, LS1, Northstar have very stout blocks and very tight tolerances compared to 340, 440 and older engines. I would not even think of building a 340 with those tolerances, it has to be loose to make power and live. I rebuilt my 71 Cub Cadet Koler cast iron engine and ran Mobil 1 in it, and it gets about 12 to 18 hrs a year running time and it's always against the gov 3600, 3800 RPM . I don't mow much so the grass is always high and has a big load at all times. After 2 years 40 hrs tops the oil did not feel slippery and actually felt sticky, gritty and looks dirty. Changed it out to conventional oil, straight wt and it's been 3 years and it is still slippery and looks clean. You will never convince me that synthetic is for everything.