Motor Oil

I recently purchased a used IH 2400B tractor, 40 hp, 4 cylinder gasoling engine.I think its a late 60's or early 70's model. The owners manual states to use SAE 30 wt only oil. Would it make any difference to use some of todays 15-40 or 10-30 oils.

The manual for my IH544 also contains a stern warning against multi-weight and detergent oils. I ignore it.

I am running rotella 15w40 in all mine, including a 50 H

I run Dello 15/40 in my IH 2444 gasser.
Old multi weight oils were not so good.

Multi weight oils will be OK but tractor will be perfectly happy with good quality 30 weight except in VERY cold weather .... down close to zero.

When your tractor's manual was written, there were legitimate concerns about multiweight oils in diesels. Not today; the oils have improved greatly and are backwards-compatible in most cases (e.g.: pay attention if ashless oils are required). Some older diesels, like Detroits, have special oil requirements but your 2400B will be happy with a commonly available 15W40 or 10W30 API CI-4+ or CJ-4 rated oil. I use each viscosity according to the operation of the tractor.

The 10W30 is used in a 38hp utility tractor that gets the equivalent of a lot of "stop-n-go" use. This tractor also has to start in the NW Ohio winter, so the winter "W" viscosity is important. Oil temp on this engine rarely reaches the viscosity temperature rating of the oil (210 degrees.. below that temp it's thicker and thicker oil that you need for the engine load and ambient temp costs fuel economy) so most of the time, its operating as a "thick" grade 30 or even as a grade 40.

The 15W40 goes in a high hours 826 that's used for nothing but heavy work... tillage. This engine gets hot and the tractor is only used in warm weather. It's pickled during the winter. The higher viscosity is necessary IMO because oil temp reaches or exceeds 210 degrees, plus, and at 8200 hours, the engine has loosened up a bit (a lot?!)so the extra viscosity provides a little more hydrodynamic protection with looser bearing clearances.

Single grade oils are "OK" in normally warn or hot climates, but the formulations of single grade oils don't get much attention, so they may not have the best "stuff" in them. Also, especially for a "stop-n-go" tractor, a multigrade oil will have better cold flow. By "cold," I mean at startup. The viscosity rating for oil is taken at two points, hot is 210 (100C) "cold" is 40C (104 degrees F). The "cold" viscosity is significantly higher than the hot, so the oil doesn't flow as well. Multigrade oils flow much better at these inbetween temps. Better flow is better lubrication and filtration (thick oil can cause the oil filter system to go into bypass sooner and more often).

In Texas, I'd thik a 15W40 would be just fine because of the heat and that gives you plenty of brand options. All the well known brands are good. Synthetic probably wouldn't be a cost effective measure for you, but Shell Rotella-T, Delo 400, or Mobil Delvac ar three readily available brands you can find reasonably priced (among a few others).