92 f350 diesel StArTer issues! sometimes works sometimes doesnt

hi i have a 92 f350 diesel motor and sometimes my starter works and sometimes it just wont start almost like the battery is dead

usually after i drive the truck it doesnt startfor a bit!

does this mean i should get a new starter i see some oil on the starter but i dont see where its coming from

should i just grab a new starter or should i get it tested
ive replaced a couple starters before but i dont like waisting money!

also is the pepboys boshe brand good? they are 199 bucks the autozone has a duralast for 149 bucks with lifetime guarantee!