You'll get about as many opinions as people that respond to this thread.
10W-40 doesn't have any higher tolerance to heat than 10W-30. Modern 10W-30's tend to have no Viscosity Index Improvers, while 10W-40's tend to have "some". That could make 10W-40 actually MORE susceptible to heat. "Thicker" doesn't necessarily mean "better".

BTW a JD 125 is a skid loader.
Sure it isn't an L 125 or LA 125?

I believe the best buy you will get for the parts to service the JD tractor is the one from the John Deere dealer.

JD has a service package that contains the oil filter, air filter and foam sleeve, 2 qt of oil, spark plug, and gas filter. Last year it was about $40, about $35 with coupon.

For several years I have checked the individual prices at the big box stores and others. I never could find a better price than the JD dealers.

Buying the service kit from JD removes the part search and what is best for the tractor. The JD dealer asked you what tractor you have, looks it up in the computer and hands you the box with the recommended parts. could not be simpler that that

You can remove the deck, grease the fittings, check the belts, sharpen the blades, and install the new parts on a Saturday morning.

You can use your manual as a guide. If any of your belts are cracking or splitting now is a good time to replace them .

This post was edited by knuttle on Mon, Nov 18, 13 at 16:27

All good notes above. Should you just want the plug and oil , the RC12YC plug and SAE30 oil is what should go in an L series tractor. Best change the filter as well,

That is interesting. My L155 was purchased in 2001 with a 15 hp Kohler engine.

In the manual it states, depending on the ambient temperature, you should use either 5W30 or 10W 30

Additional information from the manual
Plug JD M78543 Champion Equivalent RC12YC
plug gap of 0.040 inch

You can find parts for the tractor at this URL
http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet_Alt

Manual for JD Manuals
http://www.deere.com/servletcom.deere.u90785.productcatalog.view.servlets.PublicationsSearchServlet

Trying to compare L's, LT's, LA's etc. is somewhat akin to comparing apples & bananas.
There's different models of Briggs, Kohlers and sometimes either involved.
Best that the OP actually identify WHAT engine.

Knuttle: I guess I consider your Lt155 a different animal than what an L or LA is. I could see where you can make that assumption though. As an FYI, you can use SAE30 in your tractor should you choose to do so per Kohler- provided temps are +50 degrees.

I believe I said that indirectly. You can not make any decisions about any thing mechanical if you do not know what it is.

My last sentence told the OP where to get the manual for his tractor. There are other places on line

True, but any 125 Deere lawn tractor will have a Briggs OHV. Same plugs, same oil. The kits you had brought up are a good idea as well as reviewing the maintenance schedule. My only exception is Deere puts the wrong fuel filter in the kits for the gravity feed Kohler engines. Doh!

I did not realize that Kohler made gravity feed engines. Per the manual the following Kohler engines have fuel pumps, Small and inconspicuous but still there.

CV11, CV16, CV460, CV465, CV490, CV495

The presence of a fuel pump in IPL or service manual text does not mean that the fuel pump will actually be used in the specific machinery that is powered by that engine.
Fuel pumps are shown in IPLs and service manuals because it is just easier and cheaper (on Kohler, Briggs, etc.) to show it that way than to produce two (or more) different publications to illustrate all the possible configurations of the engine.
It is the manufacturer of the machine that the engine is used in who decides if the engine will be fitted with a fuel pump, or not....based on the mounting altitude and location of the fuel tank.