For your queries regarding switching to another oil with longer drain interval.
I understand that
1. You are using Caltex Havoline motor oil 20-50W (API SG)
2. Your engine has been rebuild and its 25K kms after that with above oil.
3. Oil Drain interval 3000K
4. Oil Filter Guard
5. No oil decreasing issues
Consider the following, b4 moving ahead
Engine Oils Like ZIC A, Castrol Magnatec, Shell HX7 are all API (SM) certified and SAE 10-40W viscosity grade oils . (I am not going to talk about fully synthetics like ZIC XQ, Mobil1, Shell Ultra etc as they are all very expensive and also not compatible with our 11+ year old khybers)
The above mentioned oils falls in the category of Semi-Synthetic or Part-Synthetic category.
API SM is the latest certification for oils by American Petroleum Institute. These oils are fuel saving, superior cold start protection, longer lasting and most important have strong detergents to clean and keep cleaning the engine till the next oil change (5K Kms MAX). The detergents in these oils will remove deposits (if any) from the engine which in older engines running on mineral oil might result in decrease in compression or cause temporary/permanent oil decreasing issues. In rare cases the engine smokes out. As I dont know what kind of overhaul has been done and what is situation of deposits in your engine so cant say anything.
Another important factor is the oil filter, these oils will need superior filtration as they are working to keep the engine clean by picking up the deposits so naturally they need some place to throw away the dirt. Therefore a high capacity oil filter like VIC and leppon will be required to benefit from these oils. if you dont use such filter then these superior oils are going to become dis coloured and ineffective, by picking up deposits, sooner than your previous mineral brand.
My Suggestion in order of priority:
A- Keep using the Caltex havoline motor oil (20-50W) with local filter and 3K kms drain interval.
B- Switch to Ultra 7 (20-50W) API SL certified with VIC / Leppon oil Filter and 5K kms drain interval.
C- Switch to ZIC HIFLO (20-50W) API SM certified with VIC/Leppon oil Filter and 4K Kms drain interval.
Read this post atleast twice and then decide.