I've had recommended to me recently by some other B, C, and CA owners to move up to a straight 40 weight if your engine is a little clanky, like mine is. Right now I have straight 30 weight in it. Oil pressure starts out great in the middle of the gauge, but eventually peters down to the "low" side after it's fully warmed up and hovers right on the line between "low" and "normal." It is an aftermarket gauge. My old one always read higher and had numbers, but something blew out inside it and oil filled up the inside of the gauge, so I got a new one from Steven's Lake Parts and it's always read lower, which makes me think it's one of those automobile ones that's meant for higher PSI (30+) cars that just has an "Allis-Chalmers" plate stuck in it for looks.

Regardless of whether I have a dud of a gauge, is it crazy to use Straight 40W in the shoulder seasons and summer when it's above 60 degrees F? Want to add that part that I understand winter multigrade oils and how much damage you can do using a straight weight oil below 32F, lol. 

Alright! Commence keyboard flogging!

15 psi is NORMAL at high idle if the engine is in good shape and filter is good.. Pressure with a NEW filter might be a couple psi low untill the filter becomes soaked, and gets some BACK PRESSURE.... The filter and the 1/4 inch tube inside it are what causes the BACK PRESSURE on the gauge.... thinner weight oil could actually be pumped thru the orifices easiler and get you BETTER FLOW to the bearings and spray at the piston wrist pins..

Heavier oil will cause more back pressure in the filter and make the PSI reading go up.. It should also cause higher viscosity in the 85% of the oil going to the engine bearings / pistons... That might not get you MORE OIL, due to the THICKER VISCOSITY....

I would prefer a 10w- 40 of 15w- 40 oil if you think the 30wt is too thin.

I have seen more that one B engine that  runs fine with 10 psi at high idle and 5 psi a low idle..... when you loose pressure on a B-C engine, its normally due to oil pump wear on the end covers... Too much thrust clearance and your loosing efficiency.

If I were planning to run one of these engines hard for long periods of time in 80+ degree weather doing something silly like plowing for 8 hours straight then I would probably run straight 40 Wt HD diesel oil. For the light duty putt-putting for which I actually use my Model C tractor I use only 10W-30 in the summer and 5W-30 in the winter. My engine sees lots of short runs and I want that oil to get circulating as soon as possible.

As mentioned above, these engines use a bypass-type oil filter. If you want to see higher oil pressure, don't change the filter. Did you know that filters actually do a better job of filtering after they start to get a bit dirty? As long as the filter continues to get warm you know that oil is still flowing through it and it is doing its job.

  To properly understand motor oil viscosity labeling, Be aware the _ _W number it what it will flow like when it's cold and the other number it what it flows like when it's hot. 

Soooo with a straight 40 weight oil you get the cold and hot properties that a 40 weight oil specifies. At cold temperatures 40 weight will flow very sluggish and since about 90% of engine wear is from start up until it reaches operating temperature. Do you want to add to an already worn out engine?

  Granted a 40 weight oil will help with a rattling lower end, However the best thing would be drop the oil pan and take out a couple of shims from the rod bearings. Short of that try a 10W40 or 15W40. I personally use 15W40 in everything I have, makes it simple, 1 oil for everything.

resurrection20 wrote: ... Now I'm wondering what straight 40 wt was designed for.

It's for extreme conditions such as "planning to run one of these engines hard for long periods of time in 80+ degree weather doing something silly like plowing for 8 hours straight".