Is it possible to melt HDPE to a point where it has similar viscosity to engine oil?
You may see a small reduction in viscosity with increasing temperature, but at elevated temperature the polymer will start to oxidize and/or undergo thermolysis. If I were trying to reduce viscosity I'd try blending in some low molecular weight polyethylene (LDPE). Not sure if you have the capability of doing this or if you could tolerate the resulting changes in property (loss of tensile strength, etc.). This reference:

https://doi.org/10.1080/03602550600609663

suggests you may need to go as high as 40%+ to get a significant change in viscosity.