The loader isn't helping your rear tires get traction, because it's taking weight off the rear axle. Now, a similar HP 4wd tractor with a loader is a whole different animal from an 8n. Superior in almost every possible way. That said if your only reason for upgrading is for better traction, I'd first consider loading your rear tires (unless of course they already are). You could also add weights to the rear wheels.
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Agreed
One of my internationals has a loader and front weights on it because it carries a lot more weight out back with the backhoe most of the time. Now if I take the hoe off and try to pull a plow you will quickly find it becomes traction limited. Take the weights off the front and put them on the rear axel and it starts to gain traction, remove the loader and it becomes unstoppable.
Like an other poster I believe 4 wheel drive gets over rated in a lot of instances and knowing how the machine is supposed to be set up for the task in many cases will make the need for 4x4 much more limited than most realize. Proper ballast for the task at hand rather than the generalization that this is the best set up for overall One must also understand that when the 8n was designed it wasn't a world of 4x4's out there so we took the time to understand the physics involved and moved the weight to match the task at hand.
Simply put in the old days we didn't have the option to go buy a 4x4 so we took the time to add extra weight to the rear of the 8n to compensate for having the loader or we took it off until it was needed. which ever was the easy thing to do.