This is a difficult one to answer.....
The Nissan studs are a fine thread making them a little stronger than the studs, lets say on a Cruiser. By adding the 6mm spacer one is effectively reducing the amount of thread that is going to be in contact with the wheel nut. The studs are fairly long on the Patrol, and combined with the finer thread the 6mm that the spacers take up, there is still more than enough thread to hold the the wheel securely to the hub. The spacer also fits flush against the hub and the inside diameter of the spacer is sitting just where the radius on the hub ends.
If one fits this spacer to the 30mm spacer you already have on, my concerns are that there is no center spigot for the spacer to "center" itself on and that means all the wheels forces are on the studs. Those studs, I assume, have a normal thread and not as fine as the Nissan studs, creating a potential weak point ( in my opinion ). There is in effect a slip surface between the 30mm spacer and the 6mm spacer and again between the 6mm spacer and the wheel rim, not counting the surface between the hub and the spacer and assuming that the 30mm wheel spacer fits snugly around the radius on the hub and cannot move in any direction.
I don't think it would be a good idea in my opinion to do this for your application. It is a general rule that the studs now have to work even harder as soon as you put a spacer in.... the wider the spacer the more stress on the studs, as its now only going to be the studs holding the wheel center and tight against the hub. Maybe someone else can also comment on this.
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