My friend left me about 1 quart of 5W-20 oil and I was wondering can I mix that with 5W-30 for my Tundra? Any oil experts out there think this is okay to do? It's Mobil 1 Synthetic and that's all I run now since I have over 100k on my truck. I don't want the 5W-20 to go to waste! Thanks again!