Blind leading the blind?
30 weight is too thin if you plan to drive it hard.
10w40 or fully synthetic 5w50 (mobil1, Motul, Royal purple).
Btw 1998 wrx & 199 wrx use completely different AFM's
1998 & earlier = Unisa Jecs 1 piece unit (90mm)....Looks similar to a Z32 but trust me it isn't (A Z32 is about $850 new now).
^^ This type is more resistant to oil contamination but is 2.5 x the price of the later type to replace....Be warned!
99-00 GC8 Unit with replaceable sensor element (wrx 80mm, ver 5-6 sti 90mm):
