I believe the 08 Outlander XLS(18" inch rims) uses a 38mm offset. I can get a set of 08 Lancer 18" rims with the 5x114 bolt pattern, however they're an offset of 46mm. Is this an issue?

 

My full size spare on the outlander is a Lancer rim and I have had no problems when I used it. Mine are 16s but should be the same.

 

In general, Higher offset makes wheels go in; lower offset makes wheels poke out.

So, assuming that the rims are the same width, the difference in offset would just make the lancer rim sit further INTO the wheel well by 8mm... not a huge difference, and there is plenty of room back there, so it should clear everything just fine. I would only be concerned that it might look a little funny if you're a big stance nut. in theory it might make the car handle a little worse and be more susceptible to tipping over, but in practical everyday situations, you probably won't notice much of a difference.

References:
http://www.1010tires.com/wheeloffsetcalculator.asp

 

Thanks for the info. I think I'll pass and get some nice aftermarkets.