Well 23 years ago is not that long ago. I suspect we did have different oils for gas engines and diesels.
I gas engine oil is designed dramatically different than an oil for a diesel. The primary goal of the diesel engine oils is the ability to suspend carbon particles from the diesel fuel. Gas engine oils do not care about this capability what so ever. Some oils back in the 80's did have both gas and diesel ratings; today I would guess that their is very few that can run in both
Over the years our diesel fuels have changed dramatically which is why we have different oils today. The EPA is controlling emissions much more today that in the early 80s which means the oils have needed to change.
The tough question is whether the engine oil should change at this time. Most people say if it isn't broke don't try to fix something. I don't think anyone can tell you today if a change would be harmful .
I think my recommendation is to look very hard for an engine oil that has some type of C classification on the API donut. I think our 0W40 made by Petro Canada has that both classifications today. 5W30's will not have a C classifcation. I use to see a few 10W30's with the C, 10W40's typically not, 15W40's do not usually have the S classificaion. Then you primary in to the bike oils or the synthetics - I would guess that their are some synthetics that will have both.
The change should be monitored closely. I've seen quick oil consumption upon a change but often it will settle down after a couple quarts. If it does not switch back to the old oil for a change or two and then try again.
Good Luck.
Garth
