I get the oil sampled at 19k miles after a filter change (1k before the next is due). So far so good on the reports. It's still very good for service. With the bypass setup (under the driver's door attached to the frame) there is about 8 feet of 1/2" ID hose plus the filters hold a lot of oil. Base oil capacity is right at 4 gallons then add the extra area and it's almost another gallon of oil. So every time I change filters I add almost another gallon of new oil.
As far as it looking "nasty"....... it's a diesel. Even with a complete oil and filter change the oil is black after a 100 miles. Mine doesn't look any different after 100k. You can't go by looks alone, that's why I get it sampled to verify it's still serviceable. Syn oil doesn't really "go bad". It gets dirty and that's what a good bypass filter setup will do for you, clean the oil. I know a guy with almost 400k miles on his 7.3 running the bypass and I just copy his service interval and all is good. Yeah I know it kind of freaks people out when I tell them I don't change the oil but filter it real good
. I also use Amsoil's diesel additives as well and their grease.
And yeah I'm a "perfered customer". I generally only order oil once a year. I run it in my wife's car, 25k miles between changes and that car has 168k miles on it. I also run it in my scoot, 10k on the crank and 5k on each the tranny and primary.
Since I'm one of their "perfered customers" I also order products for friends as well so they don't have to spend the $20. I don't charge them anything to order for them, what I pay is what they pay. Problem is they will only ship to my address. I've tried having them ship elsewhere but they keep messing that up and ship to my home addy.
This dude has a LOT of info about Amsoil on his site here
. And here
is his page on the PSD and bypass setup.