Gary, I can not say that mixing Volvo Pemta oil and T4 Rotella will do damage to your engine. It probably will not. Why do it for a few extra bucks. Most oils are blended oils made with similar formulas. Each oil company prides themselves with the better formula. Shell Rotella® T4 Triple Protection 15W-40 uses, synthetic blend formula exclusive to Shell. It is not a full synthetic but a blended synthetic. Volvo Penta being Volvo Penta does not give much information about their oil blends. If Volvo Penta oil is a blended conventional oil blended with a synthetic oil you will probably still retain the benefits of this oil. If the Volvo Penta oil is not a blended synthetic oil you will dilute the benefits of the T4 Rotella synthetic blends.
Bottom line Rotella T4 oil is not really that expensive. I purchase it at Walmart for under $20 a gallon . The Volvo Penta oil is pricey. You could just keep the Volvo Penta oil for emergency or give it away for the savings you will have by using the Rotella T4 in one oil change. Or buy the needed amount of Volvo Penta oil for a last oil change. At an equal cost of using all Rotella. That would be my choice, give it away or purchase enough to do one last change with Volvo oil. There is an old wise tail never mix oil brands. I'm not a chemist, I'm an old school DIY that has always followed the rule do not mix oil brands. Not because I feel it will damage the engine. I don't mix them because I don't no if I am diluting part of what makes each brand different. We pick a brand based on reviews. What makes one brand better than the other?? Something must!
