Oil filters may be a bit cheaper but I would not trust one as a final filter so not that cost effective.
I use a Fleetgaurd FS1000 as my final pre-filter as well as my truck filter. 10 micron rating at 98% effeciency. Check a Peterbilt dealer, as they have the best prices I've found. About $20 for a filter base and $13 per filter. Usually cheaper in bulk too by about 15-20 percent.
Prior to my FS1000 in my prefilter setup I am using a cheap, $16 GE water filter from Home Depot
GXWH04F . Available at the stores I've checked but not online. Lowes has a similar one, but twice the $$$. I use the foam looking filter element, 5-15 micron rated,
FXUSC $8 for 2 at home depot. It's life can be extended by wrapping with bounty paper towels, approx 15 micron near as I can tell.
