bob...i have used the gulfcoast paper towell filters on our boat, for oil/fuel for abt 15+ years...i called charley [the owner] many yrs ago, and he said to change the filter at 200 hrs, and then the oil at the normal hrs per year which is 400/600hrs...[alaska takes more hours]...the 1st 2 yrs i sent samples in at 200 hrs, to set a presedent! samples always came back as excellent! so now i change the oil [volvo 70B/6 gallons] tranny [twindisc 506] yearly too[400 to 600hrs]...and the gfc fuel filter at the same time...my 92 dodge/cummins pickup has a frantz [toilet paper] oil filter...i use scott 1000 paper, change it every 2500/3000 miles, and add what is needed, at first sent a sample in at every paper change...i change the oil [mobil/exxon #30] every 25,000 miles! sample says leave it in, but i change it! in my research i`ve found 10/30 is the best multigrade...15/40 has to much expander, etc, and less than 60% lubricant! normal autoparts store screw on filters are filtering at abt 28/30 microns, the paper towell/terlit paper filters are in the area of 1, to 1.5 microns...my research has also told me that most of the damage/wear and tear is done by the silicon/dirt that gets together and it`s at abt 5 microns that does the dirty work/wear! so, when the basic oil is cleaned down to 1+ micron, the wear is much less! with synthetic oil, if one uses the previously mentioned filters, and can go 20, or 25,000 miles per change, [with tests to set the time/mile limit] then the synthetic oil is not so expensive!...i would add that amsoil, and or mobil one, [in my estimation] are excellent oils! [i still like 10/30]...when oil is added at filter changes, it replenishes the detergents, etc...acid is formed by the mix of water, and sulfer..those by-pass filters absorb the moisture....enuf for now...c clyde fernortner garbage7@wavecable.com