I changed the gear oil out of the transfer case last week. I used a hand pump and pumped it out through the fill plug. I refilled it with 80-90. I went trailing through some wash beds Sunday, and the t-case slipped into neutral a few times on me. If I let the rpms go back to idle, you could hear it slowly start to grab back in, and then it would catch. It didn't start doing this until after I had to put it in 4. I was in regular old 2wd when it would happen, and it seemed like it was when I'd hit the skinny pedal a little to climb or get through something. It was hotter than a firecracker out. It never did this before the fluid change. Do I need thicker gear oil for the summer heat?