I have had a few Caterpillars over the years, just playing and collecting them.  I've had 2 D2's, 4 D4's one of which was a tail seat, and I just recently sold my D4 with toolbar blade and ripper as my son's have no interest in this stuff.  Long story short, my mother is still living at 93 plus and has a farm that the former tenant let trees and shrubs encroach the fields.  I was lucky enough to find a real nice one owner D8 and got it bought and delivered to my mom's farm a couple weeks ago.  So for the foreseeable future I am back in the CAT business.  I am really out of my league when it comes to these big machines, even the D4 was big to me, so I appreciate all the help from members of this forum.  When I had decided to sell my D4 I let me memberships lapse, but I've since re-anted at both the chapter and national level. Here is a picture of my new dozer. I am doing a full service of all the filters, new Rotella 15-40 in the engine and pony, new antifreeze and distilled water, new fan belts, etc. I even talked to my mother and she agrees we need to build a shed to keep it in so it will be under cover in about 2 weeks. It had been shedded all its life so I see no reason to put it in the elements now.
 

John, I'm glad to be back as a member. Like I said I am very lacking in my knowledge of these old machines so I know I have at times asked some of the more dumb questions. I'm looking forward to visiting this site more once again.

I would not use the Rotella unless it is for the "older engines". Go with a 30W diesel oil or something like Kendall Super-D XA 15-40 or similar.

Without getting into the Petroleum Engineering debate I would consider using a straight 40 wt diesel oil as closer match to the original series 3 engine oil.

Guys, thanks for the information. It is a 1955 D8 13A. I've already changed the oil and filters with Rotella 15-40 T6 synthetic oil but if that is not recommended, I'll use it to flush the crankcase and then change it out for the above. The other fluids look clean, no water, metal, etc., so I just plan to change the transmission filter twice as quickly as recommended. If you think I should just go ahead and change all the fluids, let me know. Thanks.

If the oils look good and are proper for the application I'd use what you have with one exception. I'd change out the oil clutch oil and use the current spec TO-4 to get the best clutch life.

Cat 30W Transmission Drivetrain oil works good for the oil clutches, also steering clutch and powershift transmissions. Available in 5-gallon pails.

nice looking machine. there isnt that great a difference everything is just bigger and heavier. there are now copies of the 3 main books for most machines in the tech library. surprisingly cat will still supply reprints of manuals for these older machines. i had a look at the cat publication website on the suggestion of John Gaunt there is a post in the announcements section of the forum which has the link. it surprised me that they still had info for 1920s era machines.

as for oils I wouldn't use a synthetic or semi synthetic oil no need with these machines in retirement. a good quality mineral oil is just fine and cheaper too.

Looks like someone has been a bit slow backing off pushing a tree, roots have come up and pulled the bottom radiator guard out, just a convenient spot for a stick to get up into the radiator, I would fix that before working the tractor, radiators are expensive. It is otherwise a very tidy looking tractor.

Wombat