If it's running fine it should be fine.
But it's not too expensive to replace.
Make sure you get a denso or NGK.
for best prices and good service, go to www.sparkplugs.com... my bud got one last week from them and said the customer service was fantastic.

89 4Runner
3" BL, M/T locks, 33"bfg mt, bilstein, Kayline, tubebumper, toyotafiberglass panels
TBI: Elocker,3.4 w/ORS,b+b,S2Sstg2cams,arias pistons,P+P intake,TRDs/c,URDpullies+7th,downey headers,MAPECU2,WEGO WB, SupraMAF,walboro255,stg4clutch, EPaOo2 sim