Gjac - Mine is a 2-valve 305 HP + Banks PowerPack and a tuner.
I run full synthetic, changed every 8-15,000 miles determined by amount of short trips. I do make sure to use a good filter as well, usually NAPA Gold. I did run synthetic blend up until ~180,000 but it got noticeably noisier on cold starts after about 5,000 miles, and I had trouble meeting that interval, hence the switch. As a beneficial side affect... Since I bought the van I would get a small puff of oil smoke at cold start about 1 in 15 starts. Since switching to synthetic that has gone away, I presume by re-hydrating the valve stem seals.
Transmission I do a full drain and fill using the "bucket method" every 100,000 miles. I use NAPA Mercon-V since I have the 4R100.
