I used to work for D-A Lubricant Co. and synthetics can be mixed with mineral oils (regular oil) without forming "gook". Many of the oils made today can go much further than the 3,000 mile "standard". The 3,000 miles standard is a warm and fuzzy standard for most drivers. Some of the viscosity modifiers (polymer) break down and this is what causes oil to leak out your oil filter or drain plugs at 3,000 miles. I use a synthetic blend in my other vehicles and can go safely 6-9K miles between drains. This was not easy for me a first since I too had been ingrained with the 3,000 mile drains.

There are pure synthetics used in "over the road" (OTR) trucks that regularly go 35 - 40K miles between drains. However, these operators regularly have their oil checked by an oil lab to check for wear metals, antifreeze, oxidation and the total base number (TBN). This will build a base line for the engine and show wear metal trends.

I highly recommend that anyone that wants to extend their drains have your oil sampled. This will probably cost about $8 to $12 per sample, but this is inexpensive insurance verse the cost of an engine rebuild.