I'm Applying for mech eng MS as an undergrad in pet eng, will it matter if i take another mech eng course my last sem or not? I'm an undergrad in petroleuum engineering applying for mechanical engineering MS in graduate school 

I have two options to graduate: 


Take 23 credits (I take 20 almost every semester anyways) and enroll in an Applied Thermodynamics course so it appears on my transcript as having taken one more mech engineering class 
Don't take applied thermodynamics and have all general electives and pet courses my last semester (which would still be 20 credits). 


I'm trying to weigh the benefits to the costs and I'm not sure admissions officers seeing one extra mech engineering course outweighs the negatives of 23 credits... but some input would be nice!
IMO, it won't matter.  Given that you're in the process of applying (which is a time commitment as well) I would recommend option #2.  

A reassuring piece of advice a PI gave me as I began the application process was:
the admissions committee and PIs you apply to work with do not look to see that you are already an expert in the area you apply to, they are looking for evidence that you have the aptitude to succeed in their graduate program.  

NB: I'm in neuroscience--not engineering.  

Good luck!