hisHmmm..... That's NOT how I interpreted the OP's post:
"Quick question, i have Aluminum rims on the all season and steel rims on the winter tires. They both have their own set of wheel nuts, steel and aluminum. I forgot to take my aluminum nuts when I went to change my winter tires and the mechanic used the steel nuts to install the aluminum rims."
The OP seems to indicate that his Al wheels have Al lug nuts. Is he under the mistaken impression that his Subaru came with alloy wheels and matching alloy nuts?
Or as I asked earlier, did the OP source his own alloy nuts?
I'm just going with the flow here......
Well, he may be under a mistaken impression if all were OEM... I've never had, nor even heard of Subaru using AL lug nuts, likely because there are downsides to their use.
The metal is more fragile, they are more expensive, they are more likely to react with the wheel studs, and their un-sprung weight reduction contribution is insignificant.
I'm sure that rationale doesn't enter into the marketing campaigns of aftermarket suppliers who sell the nuts with their wheels.
The OP apparently had (past tense) aluminum nuts (assuming they actually were aluminum) and they could have been aftermarket, but regardless, the point I took was whether the use of steel nuts on AL/Alloy wheels would be a problem, which was his actual concern.
Clearly the steel nuts can be used on alloy rims. This is also the standard OEM setup on Subies with alloy wheels.
In any case, I doubt the OP has any concerns regarding his wheel fasteners at this point.