Q. When a great-grandfather contradicts the opinion of a grandfather, who should the grandson listen to? Should one give greater honour to his great-grandfather over his grandfather as when giving an Aliya or similar?
A. On question 817 regarding if a father tells do something that contradicts the will of my grandfather (his father); we wrote: "Rema (Y.D. 240: 24) mentions the opinion of Maharik that one is not obligated to honour his grandfather. However his ruling is that he is, but the honour to a father supersedes the grandfather.
Torah Lishma and Teshuva Mehava (1: 178) explain that honouring grandparents is only rabbinical obligation while honouring parents is a biblical commandment. Chelkas Yaakov (Y.D. 135) rules that if both his father and grandfather are present and demand something from him, he should heed his grandfather, since both he and his father are obligated to honour the grandfather (See also Responsa Rabi Akiva Eiger (1: 68) for a similar opinion. Sheilas Yavetz however (2: 129) disagrees.
Horav Shlomo Millerís Shlit'a opinion is that there are many other variants to this question and additional fact information is necessary in order to establish priority. In some instances there may be no obligation to honor either one, as when it conflicts with mitzvos."
When a great-grandfather contradicts the opinion of a grandfather, Horav Shlomo Millerís Shlit'a opinion is that there is no preference and one may choose other factors involved.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit'a.