Paul:
Personally, I’d be leery of siphoning out your old oils. Depending upon the shape of your oil pan and just where the siphon hose ends up, you are probably leaving a sizable amount of old oil in the sump, along with any accumulating sludge, metal chips/filings, water condensation, etc.
That’s why an oil change should be done right after the engine is run, with the oil still hot, and why the drain plug is always located at the very lowest point of the system to totally void the system. Always change the filter, too!
The internal moisture condensation from repeated hot/cold cycles of engine use along with added by-products from the combustion process itself create acids in the aging oil that don’t do your engine any good. In aircraft engines, we change the oil every 35 operating hours or every 4 to 6 months regardless of run time to stay on top of this problem. Most boaters make the mistake of stretching this oil change out to once a year at re-commissioning (and I know some guys who wait many years to change the fluids, using low engine usage as their justification.)
Oil is a lot cheaper than a new engine, so…when in doubt, drain it out!
One more point: in aviation and auto racing, the drained oil is routinely sent to an Oil Analysis Lab where it is checked for relative levels of all sorts of contaminants and metal particles…with a useful print-out returned to the owner. This provides an excellent record of engine health over time and can readily identify any internal points of concern, since different parts are made of different metals and graphing these relative wear levels over time is a useful spyglass into your engine. These reports are cheap (…on the order of $20 - $30 per sample), and the companies will send you a batch of small collector containers along with return mailing envelopes.
I can just hear many of you saying “Good grief, what nit-picking overkill for my reliable old diesel…and hey, if it fails, I will just sail back in to my dock or mooring!” On the other hand, I can think of many occasions on land, sea, or air where I would have been up the proverbial creek without a paddle if the engine were to fail. Also, keeping careful oil change records, along with a general log of all other maintenance and repairs/replacements can be mighty helpful when you are trying to prove to a potential buyer that your boat has been well treated over the years.
You can Google this test lab subject for lots more info, but here are two quick links on it: www.blackstone-labs.com/about-us/what-is-oil-analysis and www.titanlab.com/test-kits
Barry Stott
Chadds Ford, PA