In the light of major research work carried out on the detrimental health impacts of ultrafine particles (<50 nm), Euro VI emission standards incorporate a limit on particle number, of which ultrafine particles is the dominant contributor. As Compressed Natural Gas (CNG) is a cheaper and cleaner fuel when compared to diesel, there has been a steady increase in the number of CNG vehicles on road especially in the heavy duty segment. Off late, there has been much focus on the nature of particle emissions emanating from CNG engines as these particles mainly fall under the ultrafine particle size range. The combustion of lubricant is considered to be the dominant source of particle emissions from CNG engines. Particle emission due to lubricant is affected by the oil transport mechanisms into the combustion chamber which in turn vary with engine operating conditions as well as with the physico chemical properties of the lubricant. The present study investigates the mechanism of nucleation, effect of oil viscosity and ageing on the particle size, number concentration & surface area. Endurance tests of 150 hrs duration each, using lubricants of two different viscometrics (20W-50 & 15W-40) were conducted on a heavy duty 230 hp Cummins gas engine, in order to understand the nature of ultrafine particle emissions. Particle size distribution is measured using Engine Exhaust Particle Sizing Spectrometer (TSI EEPS Model 3090). Data is analyzed on a 50 hr interval for both the lubricants. Geometric Mean Diameter (GMD) of exhaust particle for Oil A (20W-50) indicate a steady decline in particle diameter and increase in surface area over aging, indicating a higher rate of nucleation. GMD values for Oil B (15W-40) indicate a decrease in surface area with oil ageing. A comparative study between the particle concentration and the corresponding surface area helps in understanding nucleation of nano-particles vis-à-vis gas to particle conversion and particle growth through adsorption of volatile matter on core nuclei. Exhaust particle morphology was analyzed using TEM (Transmission Electron Microscopy) and it was found that the core nuclei are metals (sourced from lubricant additive & engine wear) around which the volatile matter congregate. The size range of the core nuclei varies for the two lubricants and the rate of nucleation is found to be very sensitive to oil ageing and oil viscosity. The study underlines the importance of a concerted lubricant formulation, in cylinder combustion and after treatment strategy to counter the ultrafine particle emissions from gas fueled engines.