Auto trans fluid doesn't really help with lubricity.
Used engine oil actually does help a little.
Lubricity Additive Study Results - Diesel Place : Chevrolet and GMC Diesel Truck Forums
CONTENT:
In this study we will test multiple diesel fuel additives designed to replace lost lubricity. The primary component of this study is a side-by-side laboratory analysis of each additive痴 ability to replace this vital lubricity.
In Order Of Performance (best at top):
1) 2% REG SoyPower biodiesel (at the pump)
2)Opti-Lube XPD
3)FPPF RV, Bus, SUV Diesel/Gas fuel treatment
4)Opti-Lube Summer Blend
5)Opti-Lube Winter Blend
6)Schaeffer Diesel Treat 2000
7)Super Tech Outboard 2-cycle TC-W3 engine oil (Wal-Mart)
8)Stanadyne Lubricity Formula
9)Amsoil Diesel Concentrate
10)Power Service Diesel Kleen + Cetane Boost
11)Howe痴 Meaner Power Kleaner
12)Stanadyne Performance Formula
13)Used Motor Oil, Shell Rotella T 15w40, 5,000 miles used.
14)Lucas Upper Cylinder Lubricant
15)B1000 Diesel Fuel Conditioner by Milligan Biotech