No, Yes, Yes, Yes, Yes...
With that high mileage on the '96, I would do as you've done... Works great! "If you do as you've done, you'll get what you got." Some times a tiny buildup keeps the cylinder/pistons thight. That is my experience.
Conversley, on the '04 I'd go synthetic;
- the oil stays on the cylinder walls so the morning start is not "dry"... A bad time for little engine. Con: Cost. (I buy a five quarts at Wal*** for under $20.-)
And yes, you can switch back at any time.
Regards,
H. Put it this way
Simply put Synthetic oil is much better than "Dino oil" because of its superior additive package. its the additive package that sets it apart from mineral oil" "synthetic oil. has the ability to flow much better at "extreme cold temperature start-up" there by reaching critical parts of an engine much sooner also that applies to the opposite with "extreme hot temperatures" its more resistant and won"t break down as quickly as mineral oil. also it containes more "cleaning detergent than mineral oil" which is important and vital at keeping an engine clean. also synthetic oil doesen"t tend to sludge like mineral oil so with all these advantages you can stretch oil intervals a little longer lets say. also expect to gaine a little more horespower from just switching over usually its around 3-5 horespower gaine i have personally used "Castrol synthetic oil" and have felt better accelaration and a quiter smoother running engine performance. and Oh" yes you can do the switcharoo thing with-out a problem ok.