this jar holds/will hold

Question

The Oxford Learner's Dictionary gives the following definitions for "will":

Definition 7) used for stating what is generally true.

a) Engines won't run without lubricants.

What is the difference between sentence a) and sentence b) of mine? Is a) less confident than b)?

b) Engines don't run without lubricants.

Definition 8) used for stating what is true or possible in a particular case.

c) This jar will hold a kilo.

What is the difference between sentence c) and sentence d) of mine? Is c) less confident than d)?

d) This jar holds a kilo.

Answer 1

If you're speaking about what is generally true, your alternatives a and b mean the same thing.

But in the examples c and d of the jar, you are talking about a particular jar, which may or may not have anything in it. "This jar holds a kilo." may be a statement about its contents right now, or about its capacity.
"This jar will hold a kilo." is a statement about capacity.

Answer 2

I think A and C refers to a permanent future situation or in what condition things will be in the future. As for B and D, The present tense suggests these conditions were true in the past, are true now, and will continue to be true for a foreseeable future.