In my little brain, I would think it depends on the application. In shear, as long as you can get the compression you need with the lighter nut, it shouldn't matter. In tension, you are simply reducing the whole thing to the lowest grade component.
If it were not a nylock nut, I'd be concerned that you wouldn't get the tension you need to stretch the bolt for proper nut friction.
Grade 8 or Grade 8.8? I don't think I've ever seen a grade 8 metric bolt
The correct term would be Class 8 nut for use with 8.8 fasteners or class 10 for use with 10.9.
Is Loctite an option?
Jvella36 said:now I heard if you use a nut with a lower grade then then the bolt, it is prone to break.
That is a load of poppycock. The alloy of one fastener isn't going to have an effect on the strength of another. The threads in the class 8.8 nylock will fail when tightening before the threads on the class 12.9 fastener. You would need to tighten to the torque spec of the class 8.8 part
How is this fastener stressed? In shear? In tension? If shear then use whatever you want, the nut only secures it.
If you really want to go deep into this rabbit hole Carroll Smith's nuts and bolts book is great, If you want to go deeper then start here
You can also get a copy of the mechanical engineer's handbook..... if you are really worried about it.
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