So almost a year ago, I bought a motor that was very clean from a trustworthy car guy. he said the motor had been rebuilt but never really used, we checked the heads and they were clean, I put on a new mainfold and carb and it was incredibly clean also. I brought it to colorado and have had many issues that have only ever arisen here in colorado. My main issue now is after I drive my car for the time it takes to fully heat up, my tempature begins to climb and sometimes goes up to immediately go back down. Along with that my oil pressure plummets in lower rpms and when im not going 20 to 25 at least. I have a new thermostat, the wire to the oil pressure sensor is new, I was told to use 5w-30 (or 40 I cant fully recall) Then i heard that was to thin for here so I now have 10-w40 in my car. My car is 6.6L and I noticed only 4 quarts came out and i put that in, and the oil reading was above full. So i am rather confused on that. My radiator has some leaks, and some shops told me that because it leaks, it cant cool properly and its causing the oil to thin out to much even with the thicker oil. I have tried additives to thicken it, i have found one that mostly kept my oil pressure in the normal zone but with longer drives it would still seem to act up. Could a new radiator solve this problem like I am told or is this something else? The oil pump has never been replaced as far as I know, but its never given me issues and when I start and when its all cold it shows oil pressure quiet high (also a new common thing here in colorado) but it shows pressure. I am really confused, and quite worried.