At 100,000 if you have not changed the trans fluid be for don't do it youwill end up replacing it. As for oil I run Quaker state full synthetic in all my cars. C5 vette 400rwhp 70,000 and still going z28 turbo 35,000 489rwhp and the v 71,000. But that is low miles come-paired to my dads 89 suburban qs from mile 1 to 435,000 when the head gasket went fixed it went another 130,000ish till some one t-boned it at 50mph so I use qs in every thing just keep the oil changed at 3000-5000 miles no mater what. It pays in the end. By the way gm payed for that head gasket because every oil change was documented at 3000 miles and other than starters and altnater the trans and engine were untouched at 435,000 gm wanted a good pr car story. Stock plugs our ngk-tr6 but any plug is ok for dd duty.