The cooking snobs will say that butter is better, and even my fading taste buds can sometimes detect the difference. But much of the time you can make the substitute. BTW the reason for the 7/8 figure is that butter runs about 1/8 water. So you may have to use 7/8 cup of oil and 1-2 tablespoons of water.
The biggest difference between solid fat and oil will occur when you see lines like, "Blend together with a pastry blender until you have crumbs the size of a pea. A pie crust made with oil will not be a tender flaky crust.
However for bannock making in the bush, corn oil works fine as a substitute for lard. and it's a heck of a lot easier to keep.
A comment below mentions cookie recipes. For this to be successful, the flour to fat ratio has to be fairly high. Bannock is 1 cup of flour to 1-2 tbs of fat. This is small enough that the flour absorbs all the oil.
Another signal: If the directions call for chilling the dough then there is enough fat in the mix that using oil would make the recipe way too liquid. I suspect this is the case with many cookies.
Cake recipes usually don't have as high a fat:flour ratio as many cookies, but cakes are often very fussy about ingredient ratios.
So: Test before counting on it. It does work for bannock.
1 cup flour
1 to 3 tbs fat or oil
1/4 tsp salt
1 tsp baking powder
Tiny amount of water.
If you are going to fry the bannock, go light on the oil.
Mix flour and salt and baking powder.
Mix in fat.
Mix in just enough water to make a stiff dough that holds together. It is very easy to overdo. Knead only long enough to mix.
1. Shape into long strips about 3/8" thick, wrap around smooth pole and bake over a 10 second fire for 15 minutes rotating slowly. (A 10 second fire: YOu can hold your hand where the cooking happens for 10 seconds)
Pat out into 3/8 to half inch slabs. Cook in a frying pan over a 20 second fire for 5-10 minutes on a side.
Pat out to 1/8" thick. Drop into a bucket with 2 inches of smoking hot lard in the bottom. Lift out with tongs after 30 seconds.
Method 1 will keep campers busy for a long time.
Method 2 makes bannock you can use for a meal later.
Method 3 is the fastest method, but it is dangerous.