I've only ever used conventional motor oil. I had an '89 Bronco II and put 189,000 miles on it, I now have a '94 Exploder and have 254,000 miles on it. The engines where never apart in either vehicle.

I always change the oil at 4000 mile intervals. Neither burns/burned oil. The '94 has a rear main seal leak so it looses a little. Still gets 18.5 mpg combined.

Currently using Havoline high mileage in the 94 with a Motorcraft filter.

 

Had good experiences with Red Line and Royal Purple, not so much with Mobil 1 - but that was on a high-mileage engine with 185,000+ miles. With the Mobil 1, the oil pressure would gradually get lower and lower once the engine got to operating temp. when compared to RL or RP staying consistently in the normal range toward the end of 3 mos. At least it was kind of a signal to change oil. Changed oil/filter every 3 mos., regardless of mileage driven in that period (never put more than 1,000mi./mo.). Switched to Royal Purple only because I found a place that sold it for less than Red Line.

 

I belive Red Line is still is mineral / synthetic blend, not full synthetic. Don't get me wrong, Red Line is excellent.

Not sure monitoring oil pressure is a good indicator to change the oil.

Were the M1, RL amd RP oils all the same grade? Generally speaking its very normal for oil pressure to reduce after warm up with any oil or grade. Temperature changes viscosity and thereby oil pressure.

 

AMSOIL all the way... My dads a dealer, we run it on all our cars, we run it in all the motorcyles, we run it in our race cars.... it works great

 

All oils were the same viscosity. Yes, aware of the effects of temperature on oil's viscosity - but the Mobil 1 tended to read lower towards the end of 3 mos. and next oil change. This was when the engine was at normal operating temp. Based on my observations, M1 also seemed sensitive to fuel contamination (due to high mileage/ring wear) compared to RL or RP. Sometimes the oil would have a strong fuel odor - I drove that car pretty hard, not quite like it was stolen though!

 

I have 5 vehicles I run Mobil 1 in, and my intervals for oil change are 1 yr or 12,000 miles plus filter changes in between. Can't say there is any appreciable differance in oil pressue after warm up for the whole duration. Two of the vheicles are almost 110,000 miles now and oil consumption is not much different than when they had 10,000 miles.

If you are runing really fat on fuel mixture that will dilute and breakdown any oil and extreme blow by (rings shot) adds more than just fuel contamination to the oil. So maybe that explains your observation. I would expect the mineral oil though to be more sensitive to those conditions than synthetic. Interesting.