I've done a "self-service" on my ST42 a few times now, first, an "oil change" is an oversimplification. There are a number of service items to be done when changing the oil. I do my oil every 100 hours, transmissions every 200 hours. I pull and check the Racor (main fuel filters) every 100 hours, replace as needed. Engine fuel filters every 200 hours. I also check raw water impeller, every 200 hours.
Here's my list:
1) Oil absorbent pads - You'll need several per engine to catch oil as you remove the filters, also some good rags. You'll need at least 6 more to line under your engines in the bilge after the change so that they are fresh and you can see if there are any leaks. Also, you'll need a few for the transmission oil change and the fuel filter changes. (Some you can re-cycle as you do the job). I'd allow for a minimum of 10.
2) Oil change pump. I bought a "Reverso" for my boat and installed it. Note that on the ST42 you can't easily hook-up the transmission drain plugs, so I'd just get one with a single pump and a wand. (there are bucket styles as well that would work). Start saving some gallon jugs to drain your used oil/Transmission fluid into for disposal.
3) 2x YANMAR Oil filters - Don't try to go cheap and use third party, many are slightly too large and won't fit. (I speak from experience). If you don't have a spare set on-board, I'd grab extras and rotate out. "Double check" that the O'rings are in the box before you start! MY 6LYA-STP's use Yanmar part number YAN 119593-35100-12
When removing the filter, there is a little catch basin with a drain tube under the filter. I put an old plastic bowl under the end of the drain tube to catch the oil so it' doesn't drip into the bilge. Be sure to wipe that basin out well before installing the new filter.
4) Oil - I use good quality, non-synthetic, 15w40 Diesel (make sure it says so) oil. The manual says 21 qts (5.25 gal) per engine. Even with running the engines first, and sucking out as much as I could, I've only gotten about 4 gals in during a change to get to full. (Then another couple of quarts to top it off after it runs and fills the filter). Be careful not to over-fill. I bring it about 3/4 up the lower engine dip-stick, then start the engine, let it run for a few minutes, and check again. I always keep it just above 3/4 on the stick, never full.
5) Oil Test Kits (2 for the engines, 2 for the trans) - I use Blackstone Labs.
So, for the engine's that's pretty much it. I do pull and check the engine and heat exchanger zinc's every other oil change. I'm in fresh water so they have been fine, but better safe than sorry. Also have changed the air filters and belt once a year (YAN 119593-18880 $8 ea & YAN119593-42280 $35 ea )
I do my ZF transmissions every other engine oil change (200 hours).
For the transmissions (ZF Hurth model HSW630VI 2.8) it's one filter for each (Part #LF-ZF-9031) and 5 quarts of Dextron II-D or III transmission fluid for each. Again watch for o-rings. There are TWO and they usually stick one down inside the filter (which is a pain in the ass). I hired someone to show me how to do the transmission the first time as I didn't find the filter assembly to be intuitive.
The first time, we sucked out the oil from the tranny through the filter hole, then pulled the lower drain plug. We only got about 1 oz out of the drain plug, so we don't bother with the plug anymore as it's a pain to get to. Just suck it out well.
Now.... On to the generator! Again, every 100 hours of runtime here. My ST42 has the Onan MDKBL 9kW with the Kabota engine.
Filter: Onan (Kabota) 185-5835
Oil: 15w40 (same as the engines) 4.2 quarts.
The generator oil is easy as there's a drain hose that fully drains it, and the filter points up, so easy to get on and off cleanly.
I ALWAYS check my raw water impeller with each oil change, I've had to put two in already, they don't seem to last and fishing impeller blades out of the heat exchanger (behind your oil filter) is a pain. For the $30, it's worth replacing every other oil change!
Also, I replace the generator fuel filter with every oil change. It's small and can clog easily.
The first time I had the engine serviced (when we bought the boat), it ran about $1,800 for labor and materials. A full 200 hour service (engines, transmissions, fuel filters, generator) doing it myself runs me about $750 in parts (if everything needs changing) and takes me about 4 hours. I keep a full set of the parts (except oil which I keep a couple of gallons) onboard (just in case), and replace them as I use them.
Here's my shopping list. NOTE: This is for MY engines and vintage. Check your engines before you order stuff! I find marinedieselparts.com has good prices and availability. eBay can be cheaper, but you never know what you are getting.
Part Qty Price Ext'd Part #'s
==================== === ======= ======== ==============================
Engine Oil 10 $18.00 $180.00 15w-40 Diesel (gal)
Engine Oil Filters 2 $15.00 $30.00 YAN119593-35100-12
Racor Fuel Filters 2 $8.00 $16.00 Racor 2040N-30
Engine Impeller 2 $80.00 $160.00 YAN119574-42552
On-Engine Fuel Filter 2 $25.00 $50.00 YAN41650-502330
Engine Air Filter 2 $8.00 $16.00 YAN119593-18880
Engine V-Belt 2 $38.00 $76.00 YAN119593-42280
Trans Filter 2 $40.00 $80.00 LF-ZF-9031
Trans Oil 3 $24.00 $72.00 Dextron II-D /III (gal)
Gen Oil Filter 1 $12.00 $12.00 185-5835
Gen Oil 2 $18.00 $36.00 15w-40 Diesel
Gen Impeller 1 $30.00 $30.00 Onan 132-0379
Gen Fuel Filter 1 $10.00 $10.00 Racor 210SM
Hope that helps! The first time, it can be a pain to figure it all out, but after that, it's a fun morning in the engine room!