I know, same issue here. No Rotella to be had anywhere. Ended up getting delvac for the trucks and tractors. Fully synthetic but still a 15w40 weight. I'm sure it will be fine. Cost a mint though.
My old man bought a huskee lawnmower from tsc in 1998. Never, ever changed the oil in it. At the time he was mowing close to 3 acres with it weekly. Last year the transmission finally gave out. Took the engine out of it (23hp briggs) and scrapped the rest. Engine still runs fine, I did change the oil and filter, was surprised how nice the oil looked. Dad reminded me it burns a bit of oil, keep it topped off and it will be fine.